Step 1: Tools & Materials
UK Plug with serviceable fuse
Old AC power cable (or an old HB pencil)
Previous festoon bulb (or tin foil)
Soldering work clamp (or in my case a pair of mole grips)
Step 2: Dismantle Plug
You need to open up your plug and remove the fuse holder clasps. One will have a leg attached the other a wire clamp, we'll get rid of these with the Dremel cutting saw.
Step 3: Build Spacer
I used a section of insulated power cable, but you could use a all piece of pencil. Cut it slightly shorter than the full length of the old bulb. I then wrapped the ends with insulation tape to make sure the core wires couldn't make any electrical contact.
I then removed the contact caps from the old bulb and simply pushed them on to each end of the spacer, you could use a crimper to make them more secure but be careful not to warp them too much.
Step 4: Solder & Insulate
You need to solder a short piece of wire to each end of your newly made spacer and to your salvaged fuse clasps make sure your quick here otherwise the insulation tape will melt. Also make sure you solder to opposite sides is makes it easier to mount later.
I had to twist my spacer perpendicular to my led festoon for fitting purposes I also added some insulations tape to the exposed areas on the adapter and also the socket in the car as it was quite a tight fit.
Make sure before you fit your new adapter, you disconnect the earth cable on your battery, if you slip as your fitting it you might cause a short and blow a fuse.
Note: I didn't mention polarity, this particular LED festoon must have some sort of built in +/- rectifier because whichever way I installed it, it worked.
Note: 3 years on and its still working!