Here's the advice, as given:
"I asked Michael about the legalities of leaving a gap round the boiler
when fitting units.

He said that unlike fires, flues etc. there isn't a 'standard' legal
spec. They vary and to be legal and safe it must be fit to the
manufacturer's installation specifications. If he doesn't have the
instruction booklets for the boiler he should ring the manufacturer who
will give him the info for the boiler.

Hope that helps.

Michael said if in doubt call out a corgi registered gas fitter to
check it & make sure it is safe!!! "