Rob, I think I'm going to be able to sort this. I went to service one yesterday, (should have mentioned it). Loosened the thermocouple to check it, nothing happening to pilot, still going strong. Pulled it completely out, still going strong. Went off had a cuppa, came back, pilot still chugging away.
Bingo I thought, ID. But I switched the boiler off and switched it back on and the problem resolved itself. I'm pretty sure though when I tell the client, he will want to replace the boiler/s (there's two in this place). So they'll have two working models to keep. Only trouble is I'm booked with work for the next few weeks so they will have to wait.