The mains relay I've selected is 140mW (ie about 12mA) to drive the coil, and the main advantage here is that when there is NO power on the DC side it switches the mains on.

A agree that if the inverter and the mains adaptor both are protected against back EMF then I could hardwire them in parallel, but they are both black boxes that I can't open and don't wish to break!

Your FETs idea is a good one; I'll certainly think about it again before actually placing the order for the relays.

Do you have any particular parts in mind or an example circuit?  I'm not sure that I have ever actually used a stand-alone power FET though I've drooled over the data books enough, sad git that I am!  B^>

Rgds