Useful link, many thanks. Between here and pelletheads.com, I'm getting lots of useful information, and this is also a fun way of learning, from people who know, rather than books.
Yes, you are right about the joint being trimmed of fat - I'd originally bought the brisket to either slow cook or freeze, simply because it was reduced in price, but then thought "what the heck, it only cost me a few quid, let's learn on it". The fat issue is something I had thought of earlier, but then totally forgot today, in my excitement.
What I think (almost) saved the day was that in the last couple of hours I saw how dry it was getting, so made up a foil parcel to finish it off - containing the remains of the sauce I had been using (raspberry vinegar, worcerstershire, and olive oil) and a third of a bottle of good beer. This seemed to turn it from a complete disaster to something edible (and a gravy more flavoursome than the meat!)