Is it OK to have a Hygena single fan oven plugged into the ring main protected with a 13amp fuse?

What is the amperage rating of the oven, I'm not familiar with that brand? 240VAC?

I am not sure of the rating without removing the oven from the housing. Hygena is / was a popular brand sold by M.F.I. with their flat pack kitchens. As it has come from the store with a three core flex, as you would find on a kettle and it has been plugged into a ring main and working OK, I think it is probably Ok. This has come with a new house purchase, and I'm wondering do I need to rip it out and have a 6mm cooker supply connected to it.

I would say that you need larger wire and at least a 20-30 amp breaker to match wire size, but if it's working OK now then try it for a while.