just got this back from the spark plug experts :

The standard part for the RS Turbo is quite good for nitrous use as it's non
projected and quite a cold heat range. Part number is BCR8ES. There are
stronger race plug equivalents which should last a lot longer and perform
better under load, NGK Iridium racing R7434-8 and Denso IK01-24, if you feel
you need to use a colder plug then you could try NGK R7434-9 or Denso
IK01-27, all of these have Iridium centre electrode and solid platinum
ground electrode, expensive though at around £25+vat per plug.

so standard plugs are ok then!!?!? what are they called?!?