And breaking news!
After cleaning the filter assembly and fitting a new filter, I reassembled it back into the tank and fitted it to the bike.
I then added petrol.
I then turned on the ignition, and for the first time heard the bike turn over. No nasty noises, all sounds well.
But...
She wouldn't start.
There is a healthy spark but I don't think fuel is getting through. I will have to take the tank off again and remove the filter assembly to see if I have done anything daft. The battery is flat now so I may leave it until tomorrow.