This turned out to be a rather simple procedure, but I could not find any useful/simple/accessible info on how it was all held together, so I've put together this simple guide on resizing Ikea Grundtal shelving.
In a nutshell, the bit inside the tube is just jammed in, so yank it out, cut to size then jam the end bit in again.
I'll go over it in more detail in the following steps.
Step 1: Mark Out the Desired Length
I found it easiest to put the shelf together first and measure the distance to the nearest wall stud. It can be handy to have some holes in the wall marking the full length to measure back from - nothing to do with screwing it up the first time!
Use a square to mark out the desired length across all of the tubes to be cut. The length I needed to cut was the width of my quick square so I just used it as a guide against the edge - not really the correct method of using that tool but it worked!
Step 2: Pop Out the Lugs
The lugs/nubbins/nut-thingies, or whatever you want to call them, are jammed into the tube under pressure.
To get them out you could cut the tube and hammer them out from behind, but I prefer to attach the screw through a fixed point and yank. In this case it is fixed to a leftover shelf rail that I've clamped in a vice, but any bit of metal with a hole in it will suffice.
Once attached, gently pull and wiggle the tube until it comes free.
Step 3: Cut
Wrap some tape around the tube to protect it from scratches.
Cut along your pre-marked lines. I've used my metal-cutting bandsaw, but a hacksaw with a bi-metal blade will do fine (it's just a lot slower, a lot harder and you need to be more careful about keeping it straight).
I then cleaned up the cuts on a bench sander, but some metal sanding paper on a block will work just fine if you are doing it by hand.
Step 4: Re-set the Lugs
The lugs can be jammed back in by laying them on a flat surface (curved end goes into the pipe), lining the pipe vertically above it and gently tapping in on top with a soft mallet (or use an offcut of wood between the hammer and the tube). Keep gently tapping away until the lug is fully pushed into the tube.
I tend to go overboard, so at this point I turned it over, tapped the lug in slightly with a ball-peen hammer and then gently tapped the side of the tube end to rivet it in a bit better. This last bit is not really necessary and is likely to be the point where things get irreversibly screwed up, so it's up to you.
Step 5: Clean Up and Assembly
There will probably be glue residue and tiny scratches on the tubes by now, so it is a good idea to clean it up a bit before putting it back together.
A wipe down with any old goo remover will get rid of any residue (I like the orange oil based stuff because it smells nice and I want to lick it afterwards - don't lick it).
Next, wrap some fine steel wool around the tube with one hand and spin the tube with the other hand to clean up any stubborn marks and small scratches.
Put it all together and screw it to the studs in the wall like it should be.
Optional step: putty over and paint the first lot of drill holes you put in the wrong spot. Idiot!