Economy 10:45?

This is by more a way of a question than a blog post...when does economy 7 start? Is it at 7 or somewhat later? Oh yes, and just how does it start? Is it by just an entry on your bill? Is it necessary to have ugly heaters to run it? Or does it just magically happen?

The thing is, hubby and I have lived here now for over ten years and very often, at around 10:45 (ish, I think, I don't actually check my watch) the light in the lounge seems to just change brightness momentarily. How much it changes varies, but over the years I have come to the conclusion that since its an 8 arm fitting (yes, I know that's posh!)^* that has some impact on it depending on 3 things:

1. How many bulbs have blown
2. How far the dimmer switch is turned up (may be related!)
3. How much attention I am paying anyway!

So, the other night hubby turned to me and said "Did you notice the light just then?". I replied rather breezily that I had, and I had in fact noticed it many times over the years.... He looked at me in that way that suggests I may finally be losing it and then said..."I wonder if they switch voltage or substations or something this time of night"....which made me think of Economy 7...hence the questions... 

^* ....but necessary as our ceilings are over 10ft away....and before you say anything, that's not posh either, it's just chilly in the winter!