OK folks, the latest update
having crawled around under the floors and found nothing, we took the boiler apart
the heat exchanger was very corroded on one side, and the panel was soaked. The expert with us said it was caused by either condensation in the flue, or the flue leaking water, not a leak in the exchanger
I was a bit puzzled, and thought maybe the exchanger did have a small crack. It could not be seen and we did not have the equipment to test it
anyway, belts and braces, we changed the heat exchanger and put a load of leak sealer in the system and now....no leak.
hooray