Further progress
I realised I didn't need a picture as I had a pump on the shelf
That was revealing, someone had definitely been here before. Either the suction or return pipe was placed over the breather, blocking the correct route for the metal connecting spring
Fixed that, bonded the metal to the top plate with araldite type resin and reinstalled. Well, I did after figuring out that the infamous baffle was lose, so I manipulated it into place and hope it holds
The gauge still showed 10 litres, even though the resistance was 50 ohms. I cleared SID and did a power cycle and at last Tech 2 could see an empty tank.
Put 5 litres in, still empty
Put another 5 litres in, still empty
Long story short.... after checking the rest postion of the float on a flat surface, replacement is not the reverse of removal, you have to have the alignment mark at the 90 degree position as WIS says, otherwise the float jams somewhere
So, after resetting to factory value and subject to tweaking tomorrow, I think we're sorted