Well there's no doubting that the circuit is more then likely 6mm (maybe 4mm) but 6mm it looks like, this must be verified. So where you have the 2 x 2.5mm coming out of the one cooker outlet I would change this to a dual gang accessory box and on one side of said I would have a cooker outlet plate with a 2.5/4mm butyl flex feeding the oven - I suspect the oven is around 3.2kW total..this also must be verified. On the other side I would run a new length of 4/6mm t + e (depending on what cables there) from the box and terminate non maintenance style to the island. At island position I would use another cooker outlet plate and run 4mm butyl to the hob from this. It is imperative when finished to check loop, and check earth continuity etc - also a quick trip time check would be indicated, and indeed pertinent Wendy. Also local isolation - not getting involved in all that malarkey tbqh.