These are the larvae of a scarabaeid beetle (Coleoptera: Scarabaeidae) and from this image I would plump for the Rose Chafer, Cetonia aurata. These larvae will certainly have been the reason for your failed crop of spuds, but they are such beautiful beetles I reckon it is worth it - in fact I will give you a couple of quid to buy some potatoes if you put them back
Thank you for the identification. We did put them back as we weren't sure if they were stag beetle larvae. I think we have enough potatoes to share with them so won't need to take you up on your kind offer!