David wrote:
Cheapest is about £130 (didn't check for VAT and shipping).
Wondering if a cheapo one would be "good enough".
That is, not accurate enough to certify a gas boiler after service or give
a detailed read out but to enough to discriminate

It is making a wall mounted one with a read out (not just warning beep)
look like a potential option.

I'd go with one of the ones with a display from a brand like Aico or Fire
Angel. You know those have been certified to UK regs and have some degree
of quality control behind them.

I imagine they're OK to use in 'portable' mode if you treat them as if they
are fixed - don't wander around with them, leave them resting somewhere for
some minutes to acclimatise, paying attention to the instructions about how
they should be 'mounted' (which you aren't doing, of course, but will still
affect the reading).

Theo