Sorry i haven't been about for a while, as the weathers turned nice I've been outside doing my vehicles.
The first step of the modification is to take off the rear plate then start cutting!
Cut to the red line in the below picture. approx 10mm inside the ridge where the plate sits.

Now you have to buy a light. I went to a caravan shop and bought a strip light that was 20" or 500mm wide. Obviously, make sure its a 12 volt one most come with a diffuser attached to it. It will look like this

This is where you need to use your imagination.
basically, you need this unit to sit approx 50mm - 60mm inside the hole you cut in the boot lid. I used some old angle brackets that i then bent to suit my needs.
once this is mounted, attach the old wiring from the old number plate light into your new unit.
test the unit. when you turn your lights on the unit should light up. If it doesn't you've wired it wrong or the switch that some of the units have is switched to off.
This next bit is the tricky bit, you need to go to a sign making shop, and get them to cut a piece of clear acrylic (pref 3mm thick) at 520mm x 110mm with radiused edges. then they need to cut some black vinyl with your registration cut out of it IN REVERSE, it will look like this...

Once this is done ask them to coat the entire rear of the plate with white vinyl, some places may say you need translucent vinyl, but you dont, normal white vinyl is best for this.
So now turn it round and you have a plate, white lettering on a black background. If you hold it up to a light you will notice that the white allows some light through where as the black doesn't. while in the sign shop see if they have any thin double sided tape. Sign compnies usually stock this and it is better than any you will find in B & Q. fit the tape to the edge of the plate, on the rear of course. make sure you DONT get any on the reg number.
Go back home and fit the plate into the recess and you're done!
If any one gets stuck, just let me know.
Cheers