Good news and thanks for all your suggestions. After testing the alternator it was found to be faulty. I removed it from the engine, which was a nightmare.
Had to remove the air filter completely in order to have enough space to work
The old alternator was so tight, I had to prise it out with a lump of wood
The new alternator was quite heavy which made it extremely awkward to hold while trying to get the top bolt in, but perseverance won in the end
One thing I did learn was the old unit was so rusty and near enough seized, and the culprit was the rubber seal around the channel beneath the back of the bonnet was not stuck properly allowing water to drip down onto the alternator. It must have been like this for years, so it is worth checking if you have one of these vans, as it will save you loads of money and aggravation.