Thanks for the previous replies about X10.

I'm struggling to obtain external X10 PIRs. The only "proper" ones
appear to be the MS14A, but they're not supposed to be used in the UK
because they broadcast on 310MHz.

Question is, what would it interfere with if I bought one. I've googled
a bit, but I can't find out why 310MHz is bad in the UK.

Any clues?

Thanks.