Hello.
thanks for all comments.
the plumber came back without any hesitation to fix most things i'd asked for (including, mounting expansion tank and insulating pipes, etc).
He didn't brick up the holes though. However after another phone call he has agreed to brick it up (hopefully it'll be a decent job)
He hasn't put his final bill in and he actually offered to subtract ~£200 until the brick work was done.
I have to say i was very close to asking him to do it once and then just phoning gas safe.
For those interested the total will be ~£7,800, which includes fitting the boiler for a 19 radiator system, converting hot water and central heating systems to pressurised systems (both previously open vented). the hot water cylinder has changed from a 120ltr to a 250ltr and he reinforced the floor (though i supplied the 18mm ply). The cylinder is also connected to economy severn and the whole system is controlled with a nest controller.
I got the impression he is reasonably competent and hard working, but it took him about 15-18days, with a lot of 5 or 6 hr days - guessing he was going to break downs / higher paid small jobs.