Flashing alternator light

About to travel home (in Devon) after a day out in Cornwall in our 2004 Symbol when alternator light started flashing and a relay was clicking under the driver's seat. Turned off engine and restarted and it was ok for about 10 min then problem resumed. Ended up getting the vehicle recovered rather than risking a total breakdown although still drivable. A few days later tried to restart but the vehicle battery was completely flat. Recharged it via ehu and got it to the local garage. On start up the problem resumed but stopped after a couple of minutes and couldn't (so far) replicate the problem.  The garage checked the alternator which was working perfectly and there were no obvious loose leads. Garage suggested replacing the battery as a next step but reluctant to do so without a definitive diagnosis. Any ideas please?

Identifying which relay is clicking, and what its intended purpose is, should give more clues.

But if the garage suspects a faulty battery, the first thing I would do is to have that tested with a professional test rig.

The alternator B+ output is connected to the starter battery to feed the chassis electrics and charge the starter battery. The alternator D+ output is a signal voltage generated when the alternator is working.

The alternator light is connected between the Battery + and the alternator D+. When all is well the voltage at each side of the light is equal and the light goes out. If the alternator light is on it is usually an indication that there is no or little voltage out of the alternator and the vehicle battery is powering all the chassis electrics.

The D+ output is also used to operate the split charge and fridge relays for the conversion when the engine is running. Hearing the relay click and seeing the alternator light flash suggest an intermittent output from the alternator (or connection to it or to chassis) and that being the reason the starter battery discharged.

That is v clear, thanks. I started it today and the flashing and clicking lasted about 2 min before everything went back to normal. The transient nature is making it particularly difficult and I'm looking for a pattern. The problem first appeared after I'd spent the afternoon with the diesel heater and interior lights on so depleting the charge in the auxiliary battery. Could this problem arise from an intermittent fault with the split charge relay - maybe it's sticking, either open or closed? Maybe when the auxiliary battery charge is reduced it is taking priority over the starter battery for the alternator feed? I'm not sure how this might lead to the starter battery going completely flat? It certainly seems to be holding its charge st the moment adding to my doubts that this is inherently a battery problem.