Well, that's that then - re-installed - NO leaks! Yippee
Some learnt lessons then.
1) ensure the pan exhaust is in line with the stack connector - and at the correct angle. If too low, cut out a couple of base shapes from hardboard, with slots for where the screws go through. Use as necessary to raise slightly. If too high then ??
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2) check for level pan several places - mine was level at rear but slightly out at front (made twisted).
3) Use a decent pan to stack connector. I had bought two Kwikfit plastic ones from B&Q and Focus - both leaked from the palstic bit that holds the rubber flange around the pan exhaust. Went to local plumber's merchant and bought a rubbery Multikwik - excellent! only cost about a pound or so more than the others. Others a waste - this one did the job.
4) ensure that all the seals in the cistern are sound before final fitting to the pan. Fit all the bits including water inlet and overflow - tighten bolt/washer/rubber cone as tight as it will go without forcing it; place an open step ladder on its side in the garden; take the cistern down and place on the ladder; fill with water from garden hose until absolutely full to the point of overflowing; leave for few minutes and then check all joints for seepage - tighten as necessary, wiping away seeped water so you can see if any fresh arrives. When all leaks gone, flush and empty - fit to pan.
5) ensure that the cistern sits squarely on the pan - make sure it doesn't start leaning back when screwed to the wall - don't screw it to the wall until balance horizontally with the cistern/pan bolts and wingnuts (use a spirit level), and until connected water and overflow, and filled with water.
