I think...

...I've worked out why yesterday was so expensive, why this morning cost less and potentially how to "fix" the problem.

I turned the installation off and thought that was the equivalent to turning a computer off - that nothing else would be usable until I turned the installation back on again.

I was wrong and I'm sorry for my mistake.

What I *think/hope* I need to do is to turn the heating on for an hour in the morning, say that I'm not home during the day and put it on for an hour before I go to bed which will be £1.20 a day rather than £4.  I haven't found a way to do the same thing for weekends, so I've just set it to come on for an hour in the mornings then turn off.

If it works overnight then I've solved the issue and don't need to keep my New Years Resolution on Sunday after all.

I'm just glad that I fiddled and was able to sort it out myself lol