'99 Td5 battery issues, possible new alternator?

Hey guys,
Was driving along a few days ago and saw that the speedo and fuel gauge were totally dead, I stopped the car and it refused to start again. The RAC recovered me using a booster to my battery and I've not been able to figure out the cause since. The battery light didn't come on at any point either.

I took the seat off and saw that the battery the previous owner was using was tiny (540A 70ah) so I've replaced it with a larger one. (900A 100ah) I was hoping it'd been a loose connection that caused the issues whilst I was driving but the new battery is now nearly discharged after having been fine for a day or two.

How much of a job is fitting a new alternator if req'd, and what should I be checking in order to establish whether it is the alternator or not?

Cheers!

If the TD5 is the same as the older engines, a quick check of the alternator is to measure the voltage output across the battery with and without the engine running. It should be about 12V with the engine off and about 13.5V with the engine running.

Although I have not replaced the one on a td5 I cannot imagine it being too large a job. Removing the drive belt is easy enough (have done that), and it should then just be a case of unbolting the alternator form it support bracket and bolting a new one on.