I ask this question as i have just read above that a rcd protected mcb
should not be used for outdoor lighting.

It is fine to do so, but you may not wish to share the RCD with another
circuit, as leakage may get quite high in wet weather. Personally, I
wouldn't want any exterior circuits to share an RCD (i.e. on a split load)
with any circuits internal to the building.

I've used RCBOs for this, but you could just use an MCB off the non-RCD side
and fit an additional RCD just for that circuit in a separate box (or an
entire consumer unit for exterior electrics, if there is a lot of it).

Christian.