Full story- I realised something was wrong when we parked up in the aire in A Rua on the N120 between Ourense and Astorga, the pump kept recharging the system. Discovered water seeping from a split in the calorifier insulation, so drained the system to have a dry night. Realising there was a hardware/ builders merchant on the high street 5mins away, headed there in the morning. They didn’t have the JB Weld solution but something called Pattex Nural 27, you can buy in uk. So bought that for 17.95e and headed out for Astorga (must visit place) parked up in the aire at the bull ring and set about cutting away the insulation from around the split. Nothing to be found, so filled the system, water coming from nearer to the end of the tank, so cut more insulation away to reveal 3 or 4 tiny pin holes in the end cap of the tank where it bends over the body of the tank. Each hole spurting a cotton thread jet. So mixed the two tube sealant and applied a good layer over a holed area and left to cure. Went out for an evening of bar crawling in Astorga to celebrate (all those free tapas with each beer) what looks like a good job. Filled the system this morning, put eberspacher on for an hour and two showers, no leaks, seems like a good fix to get us home clean. It’s a 21yr old motorhome so thing go wrong now and then!!
Thanks for offer of help from Girona and JB Weld idea.