Andy Wade wrote:

[lots of good sense]

To presume to summarise: 2.5mmsq is *not* against regs as an absolute
matter. But it *often* will be, and to establish exactly which side of
the go/no-go line your particular situation lies needs detailed design
calculations. Moving up to 4mmsq pushes you usefully further away from
that dividing line - though with the UK cables the earth conductor's no
bigger in "normal" 4mmsq than in 2.5mmsq, so the earth loop impedance
isn't helped away from the margin as much as one might like. Getting a
few metres of 4mmsq or 6mmsq would be the simplest way of staying with
the current circuit arrangment safely; if the 4mmsq is chosen, Andy's
further suggestion of changing the MCB for a 32A is a further Win.

However, gory stories of "2.5mm T&E will be no more than a memory and a
paragraph in the fire brigade's incident report" are just that, and fail
to distinguish between the role of fuses/MCBs in overcurrent versus
fault current protection.

With split-appliance ovens and hobs being as common as they are, you'd'a
thunk the IEE might get round to putting out a Note about wiring 'em to
existing cooker circuits. Nor would MK, Contactum and friends do us a
disservice by marketing a dual 45A switch (in both "landscape" and
"portrait" orientations!) with neat little "hob" and "oven" labels, to
fit existing deep double boxes, which would give us two sets of switch
terminals to wire our 2.5/4/6/10mmsq final-leg cable to.

Stefek