It's definitely a leak - we've found it!
Me and my mate Pete have done some more excavating and worked out that the leak is in the supply pipe, before the stopcock in the building - and hence not under the floor, in the concrete etc. etc.
The plan, at present, is to run a new supply pipe from the meter to the building (about 25 yards) on the basis that if we fix the current leak, it will probably burst somewhere else. It's old (possibly very old) iron piping and on its last legs. So, I'll be digging a trench for the pipe and we think we can get a new pipe up to the stopcock, via the route that the existing pipe takes, without too much disruption. The building is about 120 years old and the footings are, um, not very deep, so it's not a major issue. We're still debating whether to get a plumber in to connect everything up but I want to find out more about where the existing supply goes (it feeds both the outbuilding and our house) before deciding, though Pete - who's an incredibly enthusiastic, can-do kind of guy who has renovated several old houses in his time - is dead keen to get on with it, yikes