A correction.
If you did bury the cabe then it would have to be like this from the MW/K socket to WM socket.
View attachment 142666
Thanks for the correction EFLImpudence.
Don't forget, you won't get 1 x 6mm² and 2 x 4mm² wires into the FCU supply terminals, so change your radial layout diagram to suit so it's just one 4mm², you'll even save some plastering by not coming down in two different places with the new cables.
Thanks rsgaz, I'm going with the MK FCU idea you mentioned earlier, but keeping it as a 2.5mm ring, mainly because I have like, 30 odd meters laying around plus I think the walls need plastering regardless. I'm just seeing it as an extension with an FCU to what is there already. Only time in the last 9 years I had a problem, was when the RCD tripped when the oven element blew.