Its unlikely to be a mobo problem I'd say, sadly won't know for sure until you 'can' test it with something but I think its unlikely that your motherboard has a problem with both its PCIe slots. Sadly I've no suggestions of how to test it (save putting another GPU in )So managed to get through on the phone and to be fair PCS were great. They agreed with the diagnosis that it’s a faulty GPU and they will send a replacement ASAP. Chap on the phone didn’t know if they have set aside any stock for replacements so he said it could be a bit of a wait. Frustrating but these things happen.
Just hope it’s not a MOBO problem but I don’t have another GPU I can test.
Kudos for staying calm, I'm sure things were thrown (rightly so!) - but none the less, hope it gets sorted swiftly.