Diversity for a domestic cooker is 10A + 30% of the difference so
assuming a worst case of 50A (it should only be ~44A, but the oven says
20A so let's use that) we get 10A + 0.3 * 40A = 22A

Feed this into the calculator at
http://www.tlc-direct.co.uk/Technica...ltageDrop.html (after
converting to 5060W) allows 6mm to be used no problem for lengths up to
about 50m.

If you still include a 13A socket you should add an extra 5A, but
that's still fine for about 45m.

* Normal disclaimers apply: Check the figures yourself, dig out the OSG
before attempting to cook cables etc. etc.