Is there such a thing? The boiler I have to replace on Monday will have
one of its fixing holes in the hole which I'll have to block up from the
large un-fanned flue of the boiler I'm replacing.

There are various ways I could work around the problem but the ideal would
be a mortar that sets in, say, 30 mins - 1 hour.

--
John Stumbles -- http://yaph.co.uk

Things don't like being anthropomorphised.