It was a stuck nut, Chuck. Hardly rocket science. But thanks for the offer.
And actually, once I had elevated the cistern on to the new seal I was able to get a spanner to it - it still wouldn't budge so I left it. That seal appears to be working OK at the moment. As does the new main seal and the seal on the other (new) bolt. At least there doesn't appear to be water pissing from anywhere at the moment. I'll leave it an hour and see if there are puddles and then tighten some more if there are.