Here's what I did:
Tilted the tank so that the swarf would drop into the corner of the tank.
Used a 25mm hole saw and a 32mm stepped drill bit to take it out to 32mm dia. (float OD is 32mm)
Marked out the positions of the five holes for the sender flange.
Drilled and tapped the holes.
Did not use grease as I thought it better not to have grease and swarf adhering to the inside of the tank.
Deburred the hole and opened it out slightly so that the float fitted through easily.
So, some swarf and filings inside the tank as well as the blank created by the hole saw, and also the pilot twist drill from the hole saw was loose and had dropped in as well
Attached a 300mm long stiff plastic tube to the nozzle on the vacuum cleaner and was quite easily able to retrieve the drill, the blank and all of the swarf. The compartment where the sender is fitted is reasonally small, so happy enough that I could see all corners/edges/lower surfaces of the tank and could vacuum all of the swarf out of the tank.
Fitted the sender and gasket, coating the surfaces and the screw threads with STAG jointing compound.
After the STAG goo had cured, used petrol to clean out the tank. Poured it out and filtered it to check for swarf. Repeated this procedure several times, and no swarf in the funnel filter. All good.
Now moving on to fitting the tank.
Thanks for all of the advice. Much appreciated.
Cheers,
Glen