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.
I got one from Aldidl a couple of years ago and had never used it. As it was rough as f*** I'd stripped it and put in some decent grease. Today, moving a batten holder along 1 joist, I'd got the 12V Mak in, put the battery on it and then found I still couldn't get the bit in. Remembered the unused toy - it worked a treat and didn't grumble at all. Well worth the few quids it cost.