Just to conclude this thread, here's what we (my incredibly helpful dad and I) had to do:
Getting the swan neck off was, as people predicted, a real pain. It took 2 attempts - the first attempt, with smaller/less powerful tools didn't work - so if you're attempting this, my advice would be to get some serious tools! We used a fairly beefy puller tool, then heated the outside ring of the swan neck with a powerful blowtorch and whacked it on all sides with chunky hammers. After about an hour or two of that, it did finally come off - slowly - but it was a lot of work. No way I could have got it off without a puller tool.
Once the shaft was exposed, we then had to remove the rusted bearing, which was of course, fused to the shaft. Several cuts with an angle grinder finally took it off, but again, a pretty mammoth piece of work - probably took another hour and a half or so to try and avoid grinding into the shaft itself or the bottom plate. That didn't actually manage to get the rusted bearing off the shaft though. The thing that finally did it was the puller tool again. We had to lift the rudder up slightly so that we could get the puller tool underneath the bearing (we'd taken the advice of putting a bit of rope through the hole in the top of the rudder, so we were able to lift it - it never slipped out of the cup that holds it at the bottom end, though on other boats that may be a risk).
All that done, we were finally able to determine that the shaft width was 1 1/2" as opposed to 40mm, which are the 2 available sizes of F208 bearing unit, which I got here - thanks @OldGoat for the Bearing Boys recommendation - no need to go anywhere else. I then sanded back all surfaces as best I could, liberally applied grease over everything and fitted it all back together. Just got a couple more nuts & washers to find, but it's solid enough to pilot the boat now, without the tiller clanging back and forth. One day we'll repaint some of it, but - of course - not the grease nipple!