We use Puriclean powder which can be bought from all caravan accessory shops.Need to rinse thoroughly afterwards otherwise there tends to be a chemical taste for a short while.The above powder can also be used to sterilise your water pipes and Aquaroll.
Easy to answer this one, just get your grandad to soak his false teeth in each cup in turn, mine always come up like new. If you on the same site as me please call in for a cup of tea!