Pear and Chestnut Charlotte

Chestnut flavoured desserts are much more common in France, in fact it was after having a divine chestnut mousse at a restaurant that I was inspired to start using the ingredient. The problem however is finding the chestnut puree in South London, I've only come across it once in a supermarket near me and I guess it wasn't popular as I haven't seen it since - I'll be stocking up on my next trip to France.