Can anybody give me a ballpark figure for how much its likely to cost to hire a decent but not particularly fancy grand piano for a concert in the UK?

Since a great part of the hire fee goes to cover moving and tuning, the cost will vary according to the location of the concert and access to the hall. (number of steps, delivery and removal times, street parking, etc) weekend and evening moves can be more expensive. Some busy venues might want the piano off the premises directly after the concert or early the next morning.

Call around your local dealers. Google 'concert piano hire' and the location of the concert.

Have a look at http://piano-tuners.org/piano-hire/north-england.html for a few contacts.

I'm not at a loss for options, I was just hoping to get a rough idea before actually contacting any of them.

A local to me charges c.£550 for an evening's hire of a Kawai RX3.