A good Dyno cell will tie your car down get as mush info from you as poss...Mods carried out, octane of petrol used, driving style etc!
Then they should take it for a spin to see if there are any probs and what they think is wrong if any!
If all is ok they should strap it down on the rolleres tight front rear and side straps are always a safe bet, at this point you should have a boost guage, and a wideband afr meter installed for accuracy every good tuner should have these, oh obviously if its a non s you don't need a boost guage
then the most important things are fans...you just can't have enough there should be 2 big ass barrel fans and defo a third for your intercooler if its a S. otherwise IAT will go high and the cooler will be acting as a heat sink, around 20-25degress should be showing on your cooler if the fans are working proper again they should have a temp gauge for this!
then they will roller her on up and you will be scared i was the first time 135mph stationary
but now i love it and my car is set spot on!
Im not sure about the oxon area but Interpro in thornbury nr bristol have everything and i won't be taking my car anywhere else without checking it first!