Thanks very much for the condsidered replies, guys! This is certainly a great forum - I'm sorry I haven't joined up sooner!
I think the advice I'd been given must have been an interpretation of the "arange to minimize inconvenience" rule that you mention, Adam.
Thinking more about both...
We're in the process of converting our loft to a bedroom with en-suite.
I've been informed that any electrics installed as part of this will not pass a test/ regulations unless the house is upgraded to a split load CU.
I've already run a new seperate ring main, lighting circuit, and fire...