The price would be about right for London/areas, whats the parking like

Does the price include up-rating the gas pipe, depending on length and number of bends, plus other appliances you could be looking at 28mm for at least part of the run.

Does the price include a powerflush, an absolute must.

While I'm spending your money   get him to price for a "magnaclean", for around the £100.00 mark its worth every penny.

Does that price include VAT.

The 831 will give you 12.7 L/min, whereas the new 837 will give you 15.2, it may be an idea to upgrade if the price is not much more, especially if you want a decent shower. On that subject get one thats suitable for a combi.

Finally check the installer is Corgi registered, and is familiar with Vaillants.

Even if he renews the pipework it will still need a powerflush.

Gas pipe size will depend on requirements, and what you have. he will need to calculate that or I can for you if you post the details.

length to the boiler, cooker, gas fire etc, and in what order, plus the number of bends/tees required.