Got an electrician out (luckily no charge) who could not find any problem with the wiring / smart meter, yet offered no solution. Power still went off upstairs overnight. It *IS* the smart meter as the unit being installed so close to the RCD unit causes problems - that's based on reading other people having the same problem.
When the elecrician took readings it appeared "as normal" as they was no activity on the smart meter at that point. Practically, and electrican cannot be there 24/7 waiting for a smart meter signal. when he took readings there was no surge. Supposedly, when the smart meter sends / receives information the radio signal causes a surge with can cause the electric to trip. If that just didn't happen when the electrician was here it would appear as normal. I'm only using basic facts and common sense to work this out. Yes, wed did also try the "unplug everything" approach. I am familiar with that a faulty appliance can cause a trip, none of our appliances are causing the trip - we have plugged everything in, one by one, and at no point does anything being switched on cause the power to trip.
What is certain, is that overnight, the smart meter sending / receiving information causes interferance which ammounts to the RCD unit tripping.
Would be highly interested in any suggestions of what to do in this situation.