Which boiler to buy, reliable - nowt fancy....

The key thing is to follow the manufacturer's instructions fully. If a flue gas analysis isn't part of the detailed installation and commissioning instructions, then it isn't. For these Viessmanns, it wasn't.

ISTR on mine there was a statement along the line of "No need to do a gas analysis, we have set this up right and tested it already, don't fiddle with it!" (although more Teutonic obviously).

I think more recent versions of the installation instructions now say: do a gas analysis, but talk to Vaillant tech support if anything is wrong.

I dunno why so many are frightened of doing a self install. Assuming they already have half decent plumbing skills. It generally saves a great deal of money - far more than most DIY tasks that take the same sort of time.

And, of course, with a system boiler it's a great deal simpler than at one time. Mine took longer as the flow share valve was originally on the hot water cylinder. On the opposite side of the room. And I changed from open vented to sealed. Just swapping a like for like design, much easier.

I dunno if it's been replaced by some auto system now, but when I did my install, the tech help was superb. From an actual human on the phone. And I made it clear it was a DIY install. (Only thing I needed help with was adding the mods for weather compensation)