@adamtemp - at least that makes the choice simpler
@jchamier - thanks for the suggestion, I've just spoken to Solwise and they really seem to know what they are talking about. Looks a better bet than Ebay for sure.
@Andrew - for some reason EE are providing 4G mobile broadband with a 5776 at the same price as SIM only. So I thought I'd try it first.
I'd got in mind using a better router (with an ethernet port). Where I am I'll probably need an outside (roof) antenna, does the router you suggested allow connection of dual external antenna (the picture looks as though it could) ?