Bought a new relay for about £8 off eBay and it's all sorted!
Then I realised my new 'aluminium body' indicators were all flashing at the same time - worked out it was because they were earthed as I'd made up metal brackets and attached then to the radiator housing! Unmounted them, letting them hang loose to test and they're all working again
Rubber grommets on the way to insulate the mounting holes!