In Malay this fruit is called Kundang and also rembunia. The Chinese call it Pipa and use it as one of the ingredients to treat cough. The scientific name is Bouea macrophylla (Anacardiaceae).
I read that this fruit has already been imported to the UK. It goes by the name Plum Mango.
This fruit is quite expensive. The better quality ones are sold for RM35 a kilogram. I bought some as they do work in helping to ease my coughing. The best part is that they are sweet and delicious. Even the skin is edible.