Can I add, You can pick the worcester 34cdi up online for about 1150 including flue kit.
Can you ask him for a breakdown in costs to see how much he is charging you for the boiler, flue kit, stats, and labour.
He will also get to take away the old tank which will be worth 50-100 quid depending on size and weight of the copper and he'll also get to ditch the old boiler for a couple of quid so this will be more money he's making on top of what he's charging you.
If you get the breakdown in costs then drop me a pm and i'll have a look to see if its reasonable and how much money he is actually making from the works. I will make adjustements to it as to what I think is reasonable and then you'll be able to buy your own boiler (if you can get it cheaper) and just pay him his labour charges.