power in my garage/Workshop mainly workshop

[ban-all-sheds]

what we are saying here then is it will be safe in the short term if i put in a 13a fcu in,

Yup.

then as soon as i can dig up the path i'll get a new ring put in back to the house CU to supply the gar.

??

It will be a radial, as you'll be supplying a garage CU.

I will come from the non-RCD side of the new house CU, or from a switchfuse connected to the (split) meter tails.

The buried cable will be SWA, buried in the prescribed manner at the appropriate depth. Size of cable will depend on length and load to be supplied, but I'd suggest 6 or even 10mm², as by the sounds of it you only want to be doing it the once...