The horns, high and low are interlinked and have two electrical connections. Disconnect the wiring and using your meter check to see what's happening and are you sure both are working. If it sounds a bit weak then only one is functioning, if it blows your socks off then both are chirping. check those 4 electrical connectors and as Richard said the fuse. vibration from a running engine might be aiding a dodgy connection and the fuses and relays are notorious for slight corrosion. Whatever it is its going to be simple. As far as I remember there's now relay in the circuit.
Thanks Sewin - I have cleaned the copper contact on the steering wheel hub,changed the fuse, cleaned all the horn connectors (4) and the horns now work just fine. They still struggle when the engine isn't running, but as I am unlikely to use the horn without the engine running I am not going to investigate any further. It may be a poor earth but that will mean accessing the light switch in the steering column, which I will do if I have further problems.
Looking at the relays (and diagram in the box) there doesn't seem to be a horn relay.
