Money: £200 in cash will probably see you through depending on how much you spend on presents. Best take advice from the tour guide regarding money changing. When we were there you got R82 to the pound for cash in town but only R75 at the airport. ATMs were limited to a max of R10,000 and the effective rate was only about R76 when deductions were taken into account. My credit card paid R81 for hotel checkout bills.
Mosquitoes: the only place we were bitten was in the Cochin hotel bedroom on the first night. Take a knockdown spray and use it before going to bed and use deet.
Leeches: there should not be any outside of the rainy season. We did not come across any.
Food poisoning: we ate whatever was given us and did not have a problem except on the flight home through Dubai.
Camping: the sleeping mats are a bit thin, I would take a light selfinflating mat next time.
Fluids: bottled water is readily available for about 20p a litre, even on the trek, we didn't need a water bottle (hotel bottled water is about £1.50). We had beer each night on the camp provided by arrangement with the support staff for about £1.60 a 660ml bottle.
Laundry: you'll probably want to wash some things after the trek, be aware that the Madurai hotel charges double the quoted rate per item received after 10.30am for express wash and you may not get your stuff back until after 11.00pm. Don't put anything delicate in. Consider waiting until you get to Thekkady.