Yes, I do - I hadn't realised that the hot water side was still active, and it was set on "once", which explains why the eco mode was turning off in the morning. I adjusted the setting to "off", and the eco mode has remained on. Thank you for your help.
I have recently been setting my boiler on eco mode and it stays on that mode until first thing in the morning, when, at some point, the eco mode stops and pre-heating starts. I haven't been able to catch it the moment the eco switches off. When I spot it I switch it on again. What is happening...