I eventually gave in and bought an 'angle driver' from CPC.

Not actually a right angle, more like 110 degrees, but you can adjust
your drill position to compensate for that.

Elsewhere bought a set of cross point screwdriver bits, 3 flat wood
drill bits and, most important from my point of view, 4 twist drills
with hex shafts to fit the angle driver.
And was able to shorten the latter by cutting off the hex part of the
shaft just after the dimple made (I think) when they crimped the round
drill bit shaft inside the hex shaft.

So I was able to drill at right angles in a space about 4" high, and
now have a storage heater whose legs are fastened through laminate into
floorboards with a couple of M6 x 40mm lag screws (if that's the proper
description).
No risk now that it will fall over.

The gadget might be useful to others, so I'm answering my own question
here.


--
Windmill, Use t m i l l
J.R.R. Tolkien:- @ S c o t s h o m e . c o m
All that is gold does not glister / Not all who wander are lost