We normally head down to RS Tuning in leeds for an RR meet as it's one of the most accurate rollers around.
I've also used Evotune in Durham, its a lad I know there and he has a dyno dynamics rolling road so it's pretty accurate.
I could get some prices from him when I see him at the weekend, if we could get 10 cars to get their names down with a £5 deposit from each. Saves us all driving down to leeds as well.