M900 fuel problem

My '98 M900 has been stored for several (I'm embarrassed to say how many) years. It's on an Optimate, and I start it every couple of months. I keep the tank full of fuel to avoid any condensation or corrosion, and all has been fine for a long time. This weekend I went to start it, and it wouldn't turn over - I lifted the tank to check the battery and the tank was empty. Opened the oil filler cap and the inevitable fountain of petrol/oil mix gushed out.
I drained it, replaced the oil filter, refilled with oil and turned the engine over with plugs removed to circulate the oil, then started it up for a very short time - all ok. When I can get back to it I'll change the oil and filter again (the engine must be super shiny inside now)
I checked the vacuum fuel pump, thinking that if the diaphragm had failed it would let fuel through the vacuum tube to the inlet manifold. Diaphragm ok and no evidence of fuel in the vacuum tube.
I assume that the vacuum fuel tap must be not working, as fuel shouldn't get as far as the pump if it is. Does this mean that the logical answer is that it must be a float valve or float failure as well as fuel tap failure?

I've had a look through old threads, and some people have swapped the vacuum tap for a manual version - any suggestions? Indeed any other suggestions for things to check/what to do (apart from sitting on the garage floor gently weeping, as I was tempted to do)? There was a dribble of oil/petrol in the clutch, so I removed the plates for cleaning, and I'm hoping that petrol hasn't knackered the seal.....