Spurs from radial lighting circuit

Depends how many extra lighting points you are going to increase the load by. Dont forget the more lighting circuits you can split the load up with the easier it will be to trace faults or do mods. on at a later date.

It would be another 5 or 6 lighting points

In message , Quiggles writes

Yeah, but how many would there be in total?

IIRC Normal guideline for 6A circuit is 10 lighting points, assuming an average load on each one of 100W.

Personally I think I'd be tempted to keep these fed separately from the new CU

Yes, since there is no such thing as a spur on a radial - connections are allowed to branch any which way they like, and from any location (the term radial is in effect a slight misnomer). Hence you could common both circuits at the CU if you wanted.

However you need to look at firstly the total load, and secondly what makes most sense and what is going to be most useful in practice.

For loading you should work on a principle of 100W per lighting position or actual load (whichever is higher) no diversity is allowed.

From a practical point of view, it can work better if you arrange that there is still enough light to move about the place safely and get to the CU when any one circuit trips - so the traditional upstairs / downstairs split is not always the best - but you need to be mindful of doing unexpected things that could leave for example just one lighting position live when you turn off the rest of that floor. Sometimes a non maintained emergency light is a better solution for this problem.

OK thanks guys, at least I have a few lines of thought to pursue now.

Cheers!