I think I might have discovered the issue. Last week I installed some updated firmware onto the router...all appeared to be working...
I've just re-downloaded the firmware from the manufacturers website (using a different browser) and then re-uploaded the firmware to the router, and so far my connection has started behaving normally again. Though to say it's cured would be premature.
My wireless network is fine, network plugs are secure and the networking kit lives in a place with very little electrical kit near it.
The 6:30 restart was me, but the 12am wasn't.
Plusnet unlimited FTTC load balanced with BT Infinity 2 and BT TV