The source of the long sought leak has been identified: minute cracks in a section of grout!
Those with long memories may recall that this leak only occurred when one of the daughters visited and used the shower. We established that showering involved shaving the legs and usually emptied the header tank leading to air lock issues.
On the last visit, under direct questioning, it was also learned that she runs the pumped shower at full *throttle*! With the side panel removed and a strong jet of water directed at an area of tiling showing tiny cracks in the grout, water soon trickled out at the end of the upstand.
The shower is part way to being reassembled with shower board replacing the damaged area of tiling. Doing the whole job in shower board would be a major upheaval so my plan is to re-grout areas where there is any doubt.
However, the existing grouting is rock hard and not susceptible to the conventional grouting rakes I have. Is there another way? I have already discounted the angle grinder idea:-)
The other query is door seals. Googling gets 10^6 + 1 hits but so far, not the narrow T section I need. Someone in here recommended a supplier but I failed to take note at the time.