Ah, ok, interesting stuff. So when you say 'legally' Christian what
does that actually mean? I mean I'm aware electrics might come under
PartP or gas Corgi etc but what 'law' would cover this sort of job
please (and can it be applied retrospectively in which case loads of
the work I have come across would have the 'plumbers' in jail
quick-as-you-like!).

Building Regulations Part H.

Table 1 Minimum trap sizes and seal depths
Appliance, Diameter of trap, Depth of seal
Washbasin, 32mm, 75mm
Washing machine, 40mm, 75mm

1.15 Sizes of branch pipes serving a single appliance should have at least
the same diameter as the appliance trap (see Table 1). If a pipe serves more
than one appliance, and is unventilated, the diameter should be at least
size shown in Table 2.

Another alternative may be to use the toilet branch.

Christian.