Thanks chaps,
I'd assumed that a socket 'spurred' straight off the 32A socket breaker in the ring in 2.5 mm^w would be a no-no (A3 radials wanting a 20A MCB) hence feeding from a 6A lighting or 16A immersion MCB.
A 2.5mm^2 radial straight from a 32A MCB, if say <1 metre in length, is ok...