Steve, firstly hi & welcome

As for who does it I can only recommend OPC Tonbridge as most of the others in Kent have been a bit variable to say the least. They give TIPEC members 10% or better on parts & labour & will negotiate hard on price. Once you move out of Kent then it is Sussex Performance & several others. Northway are good but Reading is a long haul from Kent.
As for what is required then it'll depend on what they find. Typically you'll need one maybe two tensioners. Water pumps are generally every other belt change. Yup the tensioner & change should be changed but ARE NOT part of the Porsche schedule so you need to make that clear when you book it in.
Why not come to a TIPEC SELNK meeting? Linky is here:-
http://tipec.net/region_southeastlondon.php