I hope you didnt pay him the extra if he found a problem & had to spend more time OK but the gas analyser is standard kit & if he gave you a price prior to the job then thats all I would pay him .
Did he give you a sheet with the gas pressures on for the burners & input pressures if not then...
I think swbjackson as summed it up the blow off valve is cheap & the bags the best check but if you dont sort it all that new water as oxygen in it & will rust your system so get it fixed & if you getting a plumber out buy some inhibitor to put in after fixing .
Stay away from the Worcester they used to be good but not any more.
The baxi are OK if your tight on budget then BPS at Burnley are doing a deal on the ebay
http://stores.ebay.co.uk/Burnley-Plumbing-Tel-0844-88-49663
Plumbfix have a great deal on the Baxi but your plumber will have to buy...