I have a traditional deposit, 80,000 liters more or less, and an additional 40,000 liters under the front terrace.
Due to changes over the years the under terrace deposit now is also the wall of three rooms downstairs, these are bed and bathrooms. I have inherited a continuous damp problem in these three rooms due to the cold wall.
The large deposit, as far as I can tell has never been used for domestic water but filled by tanker for irrigation purposes. The terrace deposit is filled from a supply that is piped and turned on at weekends. I have had it checked and it is 100% drinkable, so we do.
There is a proposal to change to the main deposit and eliminate the under terrace one, both the water company and I find this desirable.
What precautions should I take with the main deposit, which has never been used for domestic water and not filled in probably the last 15 years?

After such a long period of time empty. It going to have to be what they call 'Tanked' That is treated with a new skim coat that contains a water inhibiting chemical. This stops the water leaching through the concrete and creating damp patches wherever its outer walls are.

This is probably what is causing your problems with the other tank. The water is slowly leaching through the 'open' pores of the concrete. The chemical is mixed with the skim coat and applied over the old concrete. Its possible that a pool repairer could undertake it for you.

Thanks for that, it does contain some water although it's vintage is unknown ! and the method of filling and source also unknown.
I was thinking more along the cleaning/purging aspect.
I know that what will be the eventual source is ok, but as for it's current contents and state ........