Thanks for the advice everyone. I take the point about giving up games, and if that's what it comes to then of course health comes first. However, I want to emphasise that I am experiencing no pain whatsoever when using the Filco. Last night when I came in from work I gave computer use a break for a few hours, and my hands were feeling fine. Then I played a couple of hours of Starcraft before bed, which is extremely taxing on the left hand (with a lot of key presses and stretching) and I felt no discomfort at all. This is compared to even 30 mins of normal use on the Ergo 4000 becoming very uncomfortable indeed. In other words, if my current lifestyle is sustainable with better equipment, that is of course what I would prefer. I am begininng to suspect that the problem has been caused by years of using MS keyboards with horrible key action (before the Ergo 4000 I used an MS internet keyboard, which from memory had a very similar feel to the keys).
Those asking what I do for a living, I am a researcher and computer programmer, so keyboard use will always be a huge part of my life. If I can get this right now, hopefully I will be set up to use computers pain free for the forseeable future. If it doesn't work out then I will certainly make the appropriate changes to my lifestyle.
hoggy, a cheap cherry brown board like the mx11800 sounds like a great idea. I can only see one on ebay though, and shipping to the UK is expensive. If I can find something like that for a reasonable price I would give it a go, otherwise I might just get another Filco, whether my employer is prepared to pay for it or not.