Why not buy a standalone PIR and wire this in to switch the lights?
These can be bought from Screwfix too (I have no connection with Screwfix,
apart from
being a reasonably happy customer).

I always try and fit a standalone PIR just so that I can keep the light and
what triggers
it as separate units.

Thanks for this suggestion. I did look into this idea once before, but
I'm fairly sure that all the ones I looked at weren't suitable for
some reason. I can't remember why though. Could you give more details
like what you wired it to? Wiring from the wall swithch? Wiring from
the ceiling rose?

Thanks

Allen