It looks like this may be the solution:
http://www.rawcomponents.co.uk/led-flasher-relay.htmlAllows a load of 30W and the front buld is 21W. Clearly hazzards will not be an option but that's fine as the car doesn't have them anyway.
Will update this if there are any problems.
If you are buying rear LEDs and have the time, the other option is: Tridon EP12 or EP 35 which is available everywhere but Europe.
Cheers
S