Yes, everything now seems to point to a weak battery, despite having had it tested.
I had the battery on charge early in the week then took it off charge. Over the next 2 - 3 days the bike was starting first time every time. Today when the voltage had dropped to just over 12 volts it refused to start. The Tune ECU readout showed the Start Relay as activated and then apart from a brief click from the solenoid it wouldn't turn over. I then used jump leads from my car battery (which I tried previously) but with the voltmeter connected it showed very little change and still wouldn't start. I then started the car which pushed the voltage up to over 14 volts and the bike started immediately. I'm now thinking of buying a Lithium Battery, which I know was suggested earlier in this thread
Lithium batteries are not kept in stock where I live (Guernsey is a small island between the UK and France) so I will need to get one ordered on Monday. I'll post a further update to let you know the outcome of this saga. Thanks