Because I run the tintop on LPG I can't use the standard Rover spec Xpart Platinum plugs so up untill now I have been running cheap NGK V groove copper core plugs.

I have just replaced them with a set of NGK Iridium plugs, although the plugs that came out were only put in 6 months back and were in good nick and had recently had gaps checked I found the Iridium plugs gave noticeably better running on LPG.
When running on LPG the slightly loss of power under full throttle conditions compared to petrol is almost completely gone.

Had the same experience with my Renault

i use the denso iridium plugs on mine, not sure if its psychological but there seems to be a difference

I found only two particular sections of road where I noticed any power loss on LPG --- one is a steep up-hill slip road onto an uphill section of dual carriageway (East Killbride Expressway) and the leaving traffic lights on an up hill gradient, in both cases switching back to petrol gave a very noticeable boost, but now with the Iridium plugs fitted the difference is much less.