Update.

I managed to get it charging again and on and off it was OK until about 3 weeks ago and then nothing.

I contacted Garmin - they advised that they do not offer a service option. What I could do is return my watch and they would send out a refurb and charge me £146.57 which seems an odd amount, but also very pricey without any details on the item they are sending or guarantee.

So I found advancedrepair.co.uk and they charged £70 and the watch is back and charging properly. Less than a week to turn it around.

Apparently gunk aside (and I did clean it) the most common issue is the connection to the motherboard and they go in to the watch and repair it.

If I get another year out of it I'll consider it money well spent.