Just thought I would update this post.
After running the P38 on this Gas system for just over a week and covering over 1000 miles I must say that the RR is now a toy I can use without feeling like a pauper when I glance down at the MPG!
The Gas system works fantasticly with no noticable power drop at all, I have had no issues with the install at all and have had the car on a ramp and looked at the installation myself and its all neat and tidy. The underbonnet installation looks almost factory.
Overall I am impressed, I have been averaging about 18mpg on gas, about 75% motorway driving doing just over the speed limit (sometimes a little more) and not really caring about being lightfooted.
This car is now cheaper to run than most normal saloons on petrol.
The only issue I have is the LPG guage, it is not accurate at all but then they never are.