This instructable comes in a UK based Metric Flavour.

In the UK, you also need building control approval for the installation of windows - you only need someone to come out and review the windows when you're done.

These are my first two, I'm not an expert and if I've done something you'd consider incorrect, let me know in the comments, but I'm pleased with the outcome and I'm now draft free.

This ible doesn't take you to the conclusion of fitting trim and doing the silicone sealant to make it all look pretty because we're not done decorating at the time of writing. I'll give you an update when it is!

The windows I'm replacing are Crittall Windows (http://en.wikipedia.org/wiki/Crittall_Windows_Ltd) named after the company, and are used in North America and other places in the UK.

They're also the biggest pain to replace, and there's not a lot of information on doing it that isn't in video form. enjoy.

Step 1: Measurement & Getting a Quote

Measure brick to brick in three places, top, middle and bottom, both horizontally and vertically.

Take the smallest of the three measurements and deduct 10mm. This allows a 5mm wiggle room.

I went for very simple windows which open on one side.

Make sure you draw and label what you want, and make sure you identify which measurements go with each window. Don't assume all the windows are the same size. My single glazed were lined with wood which had been adapted to the gap.

Then, once you're done, go out and measure again. There is something called the 100mm madness where people add 100mm to the dimensions, especially if they're not used to working in metric.

Ring round a few companies and get the best price. What you need to look out for is whether the units are welded (most if not all are), that they're as efficient as you can afford (A rated). Ask for supply only.

Pop in and see what windows they offer - you might not like the style or fancy extras like wood effect. I went for bog standard cheap white PVC.

Step 2: Removing the Steel Frame

First you ned to identify your screw holes. In most, you'll see them inside the window once you've opened it.

My windows are Crittal windows, very popular in old houses. They're fixed frames and the screws are nowhere to be found.

They are there though! Under the mastic. Chipping away I uncovered the screw heads and mercifully they're all fairly well kept.

Crittall windows require you to remove the glass entirely before you can take out the screws.

Make sure you suit up - you need tough gloves, hammer, eye protection and something to catch the glass both sides. I advise you have someone stand behind you holding a sheet up so any glass fragments that ping into the house hit it and fall to the floor. My first window required a lot of hoovering.

I used masking tape over the window to hold the glass together a bit. This helped stop it dispersing.

I did this on a ground floor window. It's best to start smashing at the top of the window to stop it dropping into the house. I started at the bottom and it wasn't really a problem.

Step 3: Removing the Steel Frame #2

Once the window is more or less out, you can go back and wrench the last of the glass off the screws. My windows were secured to the wooden frame by six screws, two in the top, bottom, left and right!

Tap the metal frame with a hammer (hard) and it'll eventually break loose from years of paint and come free!

Step 4: Removing the Frame

My windows were reinforced with render - that needs to come off too in this case. I cut the wood about 2/3 from the top and used a claw hammer to rip it out. It's not pretty and really did take a lot of work.

Once removed, you can put the window in.

Step 5: Realising You Measured Wrong

I didn't measure the windows, but if I did, I probably would have measured brick to brick, not render to render! D'oh, oh well!

Fortunately double glazing companies are very much used to poorly surveyed work and have extension plastic which can pad it out. My supplier did 15, 30 and 45mm extenders - I used the 15mm as I was about 30mm out, but couldn't guarentee it was 30mm all the way round.

Start by putting the cill on first, balance the window on the cill. You can use packing strips to level the window, but to be honest mine were already spot on (or near as makes no odds). Make sure the holes that secure it to the walls are in the brick, not the mortar.

If they don't slide on, you can persuade it with a wooden mallet.

Step 6: Drilling & Filling

I predrilled the holes in the plastic with a wood bit. I then used a small masonary drill through the frame to mark the wall.

Using the marks, I first drilled a small pilot hole and then used an SDS bit in a normal drill to burrow it out to 6mm. I used 6mm frame fixing screws (112mm long). Don't over tighten as it'll warm the frame and the window won't open/close.

It's best to pack the holes with spacers - if you don't have the special plastic ones, use a bit of wood or similar. This'll prevent overtightening.

Once it's fixed with screws, use a dribble of expanding foam to fill in the gaps. Do your best to scoop up any excess as it'll stick to the UPVC frame. If you do get some on it, leave it about 10 minutes until it's surface dry and rub with a cloth, it'll easily come off. Leave it overnight and it's a pain.

Step 7: Fitting the Window

Fitting the window is quite fun.

Start by removing the beading from the frame. Take the longest pieces out first, the smaller bits will fall out on their own.

Put in the window - you can use plastic spacers (free with your window) to raise it up if it needs it. It's best practice to do this to allow any condensation to dribble out the bottom of the frame though the drain hole.

Make sure you remove any plastic protectors off before continuing!

Now you can put the beading in. Start with the shortest bits. You're best off with a non marking rubber hammer - if you don't have one, go for wood or plastic. Carefully bash it in until it clips in properly.

Then put the longer bits in. They may appear longer than is necessary, however they came out, they'll go back in. Put the corners in first and it'll bow out in the middle. A few sharp taps and it'll clip in.

You can now finish off the window with a bit of plastic trim and some silicone down the smaller gaps. At this state, my windows are in and draft proof. They also open no probs. When I decorate, I'll finish with the job off, but it's best to paint under the sealant to keep it clean and fresh.

Done! Brush off the brick dust and other rubbish, remove any protective plastic then wash down with warm water (and a drop of washing up liquid).

Get hoovering and make sure you've got all that glass!