When I saw your post saying the battery voltage was 12.8, I was about to post and say it was stuffed, but Rex beat me to it. A fully charged 12 volt battery should read 13.2 volts - each cell produces 2.2 volts at full charge. That's why it needs 14.4 volts to charge - you need a bit more voltage than the actual battery voltage, so you can 'push' charge in.I tried the battery from my other bike and eveything worked fine, it started 1st jab of the button, so new battery it is. I don't understand though why it suddenly packed in and why it is showing 12.8 volts.
Rex Coil 7, thanks for the advice but I have already updated the electrics including heavy duty starter cable & battery to solenoid cable etc. I also fitted a reg/rec off of an R1 and used an Eastern Beaver fitting kit. https://www.easternbeaver.com/Main/Elec__Products/Connectors/R-R_Connectors/r-r_connectors.html
Thanks to everybody for their help.
Batteries die - often age related (they're usually not good for much more than 5 years), and possibly even more often when they're left to discharge completely. The over discharge causes a buildup of lead sulphate (a by-product of the lead acid battery), which can short out some of the plates in the cell, causing the voltage to drop, and the battery to lose it's ability to hold a decent charge. Once it's done that - it's toast.
If you have an alarm system, or something that has a parasitic draw on the battery, it might be a good idea to get a battery tender, to keep the battery trickle charged and topped up. Mine save me a few new batteries when I had an alarm.