Thanks for all the replies.

art, toggle bolts are undoubtedly the strongest drywall fixing, but IMHO they are a pain in the ass to install, especially if you have to remove the fixture temporarily.

gnomad, these Screwfix fixings may well be what I need, but previous experience of British inserts is that the insert has a blind hole, whereas what I need is an insert which will accept any length of screw, and which will allow the screw to pass all the way through.
That said, it looks as though these might indeed fit the bill,

The beauty of the American inserts is that they accept a coarse threaded drywall screw of any length, and bobad has come up with what I was looking for.

E-Z fixings, how could I have forgotten...now begins the search for a British stockist