Hadn't thought abour RCD protection (partly because it
wasn't RCD protected before). I know it's a requirement
that outside-accessible sockets are RCD protected but
does this also apply to outside lighting?

Not to my knowledge. However, the regulations are very keen not to require
RCDs in cases where they could be appropriate. In all but the most extreme
cases, the decision is left to the designer.

In this case, the RCD will be subject to nuisance trips, particularly if the
IP rating of the fittings isn't up to the job, but I think it is worth the
occassional trips to get some warning of problems.

Christian.