Vivoactive 4 won't charge

So, after 5 years of flawless service my watch died yesterday. It was fine at 4 o'clock while I was surfing (which I've done dozens of times with my watch on) but had died before I came out of the water at 5. Plugged it in when I got home, the charging icon came on for a short time (it was faint though) but it hasn't charged. Left it overnight, still nothing.

I've tried holding the power button for 30 seconds (after plugging it in), tapping it, cleaning the screen, no joy. I've tried connecting with the Connect IQ app and it doesn't connect (no surprise there since the watch is dead).

Tried cleaning the connectors, plugging it into a laptop. And each time I plug it in, the charging icon shows but it doesn't charge.

Any ideas? Anyone?