Timing is always a problem. I believe that the electricity companies are not obliged to stipulate the exact times, as they can, and do, adjust by area (or remotely) to help them manage the peak loads, especially as the off-peak hours commence.
So, the follow-on from that is that you will get, on E10, 10 hours of off peak electricity, but the timing may be different to the house next door.
I spent a couple of quid and for a Voltage Detector Stick (eBay) and then you will know for sure your timings. Just wave it near a storage heater, for instance, and it will glow nicely when the off peak electricity is switched on.
Remember the meters are unlikely to understand about BST and will operate only on GMT, so, again, best get a Voltage Stick and check for yourself, especially if “spring forward – fall back” is a mystery to you.