However, since replacing the water pump the vehicle has covered 500 miles or so without losing any water - so the fix appears to have worked so far.
I hope this is helpful to somebody.
Step 1: Diagnose / confirm the fault
The water pump is driven by the top pulley of the auxiliary drive belt (the only moving belt you can see with the bonnet open), and is situated at the LH end of the engine block. On many engines, the water pump is driven by the timing belt, making it quite tricky to get at and involving a lot of work to replace (timing belt / tensioner are usually replaced at the same time). Luckily this Corsa's engine is an exception and the pump is easy to get at once you've removed a few bits.