Anybody know how to remove the main rubber sealing washer from the fuel filler cap? The larger soft rubber seal is no problem, just open a few screws, but this one seems to be held by a snap on plastic cap which I am reluctant to put much force into in case I make bits of it. Just wondering if there is some trick to removing it.

Nope, it's not replaceable separately. I've just had to do this with mine when the nose sproinged off the filler cap. Check this:

http://www.realoem.com/bmw/showparts.do?model=0503&mospid=47865&btnr=16_0328&hg=16&fg=05

Note that item 7, which is the 'nose' piece in front of the seal and the metal shim, is no longer available. You might be able to get it in item 10, which is a gasket set. I obtained a 'good' secondhand cap assembly instead.

First time posting, Hi.

You're concerned because the 3 tangs holding the cap are really inflexible and the caps on there real tight yes?

I concenrated on just one tang and was able to lever the cap free on this one side. It's still really stuck fast on there at this stage.
Then I figure the laws of physics were on my side (try tearing a piece of paper in two places at the same time) and worst case scenario is I snap one of the other two tangs off and I already know it holds with 2 tangs. So sliding a bar longwise on the other side (under the chrome washer) and leaning on it produced a pop followed by an "omg where on earth did that go?" moment.

All tangs intact and to refit just pushed really hard til it popped back on. I was able to reproduce this on the spare I had (wearing goggles this time!)
I may be just lucky (twice) but I can warn you if you do try the same, you may bend the chrome/tin washer in the process (one did, one didn't). You may also of course, completely wreck your cap.

The gasket set I had doesn't contain the cap if that helps.

Thanks for the help. I think I will leave it alone. The rubber seal is in good condition, but the larger soft rubber seal was becoming sticky goo so I had to change that. I got a seal kit from Motorworks which has all the seals and O rings, and the chrome/tin washer. I will put the remains of the kit into "the stores".