Success! It was actually very easy and I am very lucky, but here's what happened:
To recap: The power tailgate stopped responding to the key fob and the button. After a battery reset, it would start to open (from fob) but after a few inches of lift, beeps (as if there was an obstruction) and close. After then, the power lock would not work and no buttons again.
The Fix: After some recommendations to have a look at the wiring, I popped off the trim and plastic cover to open up the wiring on the left hinge. It all looked pretty clean with nothing worn and stretched or even dirty to be honest. Now, I am not a very advanced amateur mechanic, so the thoughts of pulling out the wiring and soldering is beyond me, so with no obvious visible frayed wires, I was resolved to taking it to the Honda dealer. So i packed it carefully back into the plastic cover and closed the boot. And when I closed the boot the power lock enabled. So I tried the key fob and button and everything is hunky dory again! Hopefully it lasts.
Thanks very much to honda_saj for giving me the impetus to at least have a go. And antdad for his advice also. Saved me a trip to the mechanic and a potentially expensive diagnostic!