Sorry Nigel, I was always a little sketchy about calling solid state relays 'relays'. Their mode of operation is so drastically different they require a separate naming convention. I might use the term relay generically for any mechanical relay, but I would never call a solid state relay just a 'relay'
It'd be kind of like calling helicopters and fixed wing aircraft 'airplanes' No one calls a helicopter an airplane =) The basic operating principals are the same, the specifics and modes of operation are drastically different, as well as their capabilities.