Just an update:
After plugging in to a mains charger and leaving it for 5mins (blank screen and no sign of any life in the thing) I installed Sony's update utility and thought I'd give it one more go. I plugged in the watch (at first it tried to install driver and failed), then after opening the update programme and finding the smartwatch 2 option and re-plugging in the watch & tapping the power button quickly once, boom the computer detected the watch (it said the battery level was now 76% so that rules out a dead battery as I only charged for a few mins). From then onwards the update programme installed the relevant drivers and allowed me to reinstall the firmware. Finally managed to get some life into the watch, but, and there's a but...ever since I managed to reinstall the firmware the smartwatch kept crashing or freezing at a certain time and not responding to any inputs, no matter how many times I reset the watch, but I eventually fixed it!
I've had to uninstall all the smart extra apps and reset the watch a few more time and connect to my phone with a fresh setup and now finally things seem to be working again, but it's been a whole lot of hassle and not what I was expecting at all, let's just hope it lasts...