Had to use a Gas Safe fitter to make the final connection after I installed my own boiler last year (needed a cert. to get the Magic Boiler rebate).
He passed the system as safe even though a leak-down check had pressure falling to zero after 15mins. "Thats fine mate, they all leak a little bit. £90 please".
I investigated the fault myself and found a corroded pipe burried directly in a concrete floor. System now holds pressure for hours. did I get him back to retest it? No.
Do the job yourself but do it properly and check it afterwards. If you have any doubt about your ability or level of knowledge then get someone in.
Also ask to see their Gas Safe ID card...there are a lot of fitters signing work off on a mates Reg. number. Unlike Corgi, all gas fitters have to have their own ID now to certify work.