Hi
I'm not sure which period your after but Skytrex is the main one and I have some of their stuff and it is nice. Also you keep an eye out on ebay for Pit Road (AKA Skywave) as they make some very nice plastic terrain in 1/700th including a U boat pen. Peter makes a few pieces for their ACw range and you could also try Langtons. I've used some of their 1/1200th stuff for 1/600th as it can be quite large.
Cheers
kieran