Many thanks to everyone for the latest replies. There are too many to reply
to individually, so I hope this blanket response will suffice.

I'm grateful for all the advice regarding various types of fixings. I've
used Rawl-type bolts elsewhere on the same wall so (assuming a certain
amount of consistency) they should be secure for this job, especially given
that the TV is a small one (screen size 21") though it is still an
appreciable weight.

It might not after all be necessary to space the baseplate as far from the
wall as I first planned (50mm): some 20mm will suffice. This unfortunately
presumably means that I can't use the neat idea of bolting the timber spacer
to the wall and then screwing the baseplate to the wood: I suspect that the
available thickness wouldn't be enough for a secure grip (though please
correct me if I'm wrong).

Possibly then the best course is to use Rawl-type bolts to attach the
baseplate directly to the wall through clearance holes in the timber. Which
means I have to make damn sure that the holes in the wall are not only at
the right spacing but also dead vertical: any divergence will be amplified
horribly when the TV is extended at the end of the hinged arms.