So.... I have done some investigation. I remembered I had bought a USB inspection camera ages ago so put that behind the toilet to see if I could see what the issue was. To be honest it wasn't easy / conclusive but there were drips on the top of the pan connector so an going with the theory that it is the flush pipe. Have pushed it is a bit from behind (hardly moved) but.... The drip seems to have stopped as far as I can tell. Will keep an eye on it but suspect this is not really it. Fingers crossed.