The new cooker already has a flex connected and comes with a 3
ping plug with a 3AMP fuse (I would have thought it would be a
higher ampage but the plate confirms this).

I'm guessing a bit here.

However, 3A is far too small for the ovens. My guess is that it powers only
the gas hob ignitor and far some reason isn't internally connected to the
ovens. You may find some terminal blocks on the cooker somewhere designed to
take the high current cable for the ovens. Can you find such a terminal
block?

Christian.