Thanks Colin. I was thinking about those, although they don't look very
attractive IMO. From some rough measurements the door would not open quite
fully, but that would only possibly become an issue when occasionally
moving large furniture/appliances in or out, so temporary removal would
solve that.
Another snag is that it would mean screwing into the door. BTW, those at
Discount Locks don't come with the plastic screws they say are needed for
UPVC doors. I was hoping a cage could be fitted to the existing letterbox,
perhaps using its existing two screws under the lid.
One idea I'm considering is making up a sort of 'slide' made of cloth or
plastic sheeting and a couple of thin pieces of wood or stiff wire.
https://dl.dropboxusercontent.com/u/4019461/LetterBox-Slide.jpg
Lightweight and collapsible, stored behind a slim cupboard we have
opposite the door, I'm thinking it might be possible to improvise a simple
means of attaching it quickly when required.