Thanks
@ShaunCorbs ... I've had the toilet checked by a plumber recently when I had some new radiators installed and there was no leak - also bought the dye tablets that you put in your cistern to check for any leaks overnight but there was nothing. I remain convinced that it's not a leak as I was away for 3 days with zero water usage being recorded at all during the day - just the same scenario that I've now had for 3 years where water is being "consumed between midnight and 8am. I'm baffled. Thanks for your advice as I'm sure the answer is out there somewhere.