The indoor RCD socket is only about 20 quid, but the outdoor (single
or double) I've seen priced anywhere from 40 to 80 quid. That's
more than the pump it's going to run!

I'd use a standard outdoor socket, such as screwfix item 10562. IP66 rated
so needs no further protection. Run this off an RCD spur inside the house
that can also run an internal RCD socket if needed.

http://www.screwfix.com/app/sfd/cat/...11484&id=10562
http://www.screwfix.com/app/sfd/cat/...11438&id=14770

That's cheaper (this time) than the TLC items. Although I would recommend
both Screwfix and TLC generally, screwfix do some very good budget IP rated
electricals. The above I have and am impressed by build quality given the
price. I also have their Volex VGCKIT IP rated consumer unit which looks
excellent at any price, let alone the 25 quid they charge.

Christian.