The meter originally read 8457.39, I turned the boiler on and three minutes later the meter read 8457.48. I'm not aware of any leaks. I think I've made the right choice in contacting an engineer as clearly something between the thermostat, the boiler and the water cylinder is not working as it...
Thanks, yes it was built in 2017 but we don't have contact with the previous owners.
Just did the test and the meter increased by 0.09 (8457.48 from 8457.39), which if my maths is correct is a consumption of 19.8 in 3 minutes.
It's an Ideal Logic Heat 15 and yes that's correct - it's firing up every time hot water is used when it's only programmed to come on between 5 and 6pm.