I would avoid just ploughing up and down.
Swimsmooth.com has a load of set you could try.
An easy set to follow would be
4 x 100 warm up (4th length anything but fc)
4 x 100 drill sets of choice - catch one day, roll the next, recovery etc...
8 - 12 x 100 or 4 - 6 x 200 off whatever your threshold pace is - I'll do 100 off 2 minutes, my threshold pace being 1:25
2 x 100 warm down
going to have a look on there and try this next time i'm up at clissold.
London fields is the only outdoor heated pool i'm aware of in london. i don't think it being heated would be enough to get me out there right now though. /wuss