Hi Ian
You'll need a BA15s bulb, also known as a 382 or 1156:
http://www.ultraleds.co.uk/1156-single-white-reverse-bulbs-c-101_133_140.html
Beware though - different led configurations can vary very much in appearance, as in how they work with the reflector. If you get the wrong one's you'll just end up with a 'spot' as opposed to a lens full of light. Also - some (like those in the RS4 thread) can look yellow in the reflector when off.
The one's I currently use look more like this - and give full light:
I've actually swapped mine over with the indicator position - but they look like this when on in comparison with filament bulbs. The left side shows an led, the right side shows a PIAA Xtreme white filament bulb (so not as yellow as standard!):
If you go 'led' you will also need to fit a load resistor to each side, else you'll get the 'bulb out' warning as led's don't provide the resistance of a standard bulb:
http://www.ultraleds.co.uk/loada-solution-bulb-warning-turn-larger-bulbs-p-1213.html
Hope this helps