Voltage increases to 13.3V when engine running and revved up.
To charge your battery effectively an alternator needs to be able to maintain a voltage of 13.8 to 14.2, maximium 14.5. Below 13.8V the battery is not being charged properly so if it's only maintaining 13.3 even with a few revs then I would suspect the alternator.
Repeat the test with the engine running at about 1500rpm and load it up with lights, hrw, etc etc. Does the voltage fall even further?
You don't say how many miles your car has done, but it could just be that the brushes are worn. These can be replaced. However most DIYers are not equiped to test other aspects of the alternator so the most cost effective solution is probably to get an exchange reconditioned unit. I know an excellent place in Coventry if it's within striking distance.
Check the unloaded voltage and starting voltage again today and see how results compare. Also until you've fixed the alternator keep the charge topped up every few days.
Short journeys are also a battery killer so find an auto electrician and have the battery properly load tested. The lack of proper charging over a period of time may have done it in and winter may yet see it off. As a DIY alternative disconnect your injector loom and crank the engine for 30s. Tell us the voltage at the start and end of this test.
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