Jobe done. Front and rear speakers replaced, front tweeters disabled. Sound from the stereo now as good as the sound from the engine (almost). No damage to the car, minimum dismantling, looks are identical, and old speakers (shockingly cheap and nasty 1950s things) saved in case a future purist owner wants a totally orginal car.
Tips for anyone else:
The ebay front speaker adaptors are top, but you need to slightly drill out the screw holes to stop the plastic cracking when you screw into it. Fit the speakers to the adaptor, themn the adaptor to the car.
Watch the width of the magnet on the speakers you buy from the front - the Clarions I bought were fat, and only just went in.
The front tweeters are easy, as long as you are sure you want to disconnect them. Just knock the surround around and it will fall off and the tweeter will be loose in the door cavity. Grab it with long nose pliers, snip one lead, insulate the cut end, pad out the thread of the grill with tape, and push the grill back - you now have a bass port instead of a nasty tweeter that will spoil the sound of your nice new speakers. Do not do this unless you are sure, because there is no way to get the tweeter and the cover back together unless you dismantle the door.
Rears can be removed without a special spanner - a standard 7mm short spanner will do it. They are tough to draw out, though - I threaded wire through holes around the edge and lifted them out that way.
You can get speakers with a big magnet in here if you use spacers to lift up the new unit - if not, keep the depth of the replacements to 4.5cm. Because of this I went for bottom end Kenwoods - if I was doing this again I'd go better and fatter. Mind you, the basic Kenwoods are a different league from the originals.
Buy adaptor leads for the back (they come with the adaptor kit for the front). Otherwise there's a bit of bodging to do to make the connection.
Thanks for all your help, guys.