After draining the fuel I mixed up some 2t oil with a litre or so of petrol, and stuck it back in the tank with a dozen ball bearings.
Gave it a good old shake, and drained it.
Hardly any rust came out even though the inside looked much cleaner.
After re fitting the tap I mixed up approx 50:1 petrol and ACF50, plus some fuel stabiliser,and sloshed it about, the thinking being that the concotion will prevent any further rusting.
Before starting up next year I'll stick in a couple of gallons of new fuel to dilute the mixture.
On reflection I dont think the rust inside the tank is as bad as I thought.