I had a flat battery yesterday on my 300Tdi 1996 110CSW and subsequently found a 2.5A drain from the battery.

Removed all the fuses one by one (including the large ones under bonnet) and made no difference, still a 2.5A current draw.

Looked at alternator and the wires are bolted onto posts rather than a plastic plug - is this normal for this engine?

'Waggled' the alternator wires around and 2.5A reduced to 0.03A! Nothing obviously shorting or rubbing.

Any ideas?

Could be the diodes inside, these tell the alternator that the engine is running and can start charging. If there faulty then it's taking charge from the battery when it's not running

I would just replace alternator or do what I did on mine and Fit an isolator switch between battery and the wiring, that way i know when I turn it off, nothing is running

It is standard for these enines to have the wires bolted tot the alternator. Disconnect the wiring and check the drain again. But i am sure that you will find the alternator is ffffed.
Dont buy the cheap ones go for a denso there about £100 and i usually get a new pulley at the same time as there usually a ***t to get off