Ok so some interesting results from testing today.

I left the router where it was but removed the power connector from the power strip and plugged it directly into a socket on the wall.

Ran a trace directly to the Router IP at 0.5ms interval and only returned an average PL of 0.5% over....I was about to write "1000 counts" but as I was typing this post it started dropping packets like crazy again from 800 counts onwards (averaging 40% PL over a 60sec focus)

The data is here - https://share.pingplotter.com/SYhUaYLoRSd

I have added comments at various points to detail key points of the test such as when the power was switched back to the power strip again.

One more thing I did was also connect the tablets (running the app experiencing the problems) up to my phone's 4G hotspot for the evening. We had it running like this for ~4 hours and the app did not lock up or return the issue once!

Some good/ semi-conclusive results I think. I guess the next step is to completely replace the router and go from there. I have a Draytek turning up tomorrow so am hoping to have this installed and working over the weekend.

Thanks again for your help on this Poe!