A direct replacement doesn't look too difficult judging by the
installation manual. Has anyone tried it, and left the commissioning and
gas connection to somebody qualified?

I fitted one from scratch, on the promise that a local fitter would
commission it. But, he was always too busy, and it was getting cold.

In the end, I connected it up and it ran okay. Next step, I had a
standard landlord's gas safety check done. That checks for leaks and CO,
but not efficiency.

That boiler (Vaillant) ran beautifully for the next ten years, until we
replaced it during a major conversion.





And how much might the commissioning be?

Not enough to interest the fitter, apparently. I guess he's used to
fitting a £500 boiler and charging £2k+ for the job. I suppose he didn't
have the cheek to charge the £1k super-profit just for commissioning, so
felt aggrieved.