You have very little access to credit card facilities. Take about £500 in cash with you and convert it all into Rupees at Delhi airport. Some dollar travellers cheques are a good idea for backup. Be prepared to haggle when buying anything, but don't think that you automatically get a bargain just by being in India. You will be able to achieve a fair price for goods that you would not otherwise be able to source cheaply in the UK. You will need a variety of notes - take 500 and 1000 Rupee notes for settling hotel bills, 100 Rupee notes for most purchases and 10/20 Rupee notes for toilets etc.
It is cold on the early morning game drives, even approaching summer. Believe the Exodus travel advice about taking a waterproof and a warm layer! Comfortable clothes are important, particularly ones that can be washed and dried quickly.
Carry a small bottle of alcohol handwash at all times. A loo roll in your bag can also be a good idea for the train journeys!
If you are a photographer then take some dust covers or stuff sacks to protect your gear. And be prepared to clean the camera sensor after each game drive! You can also never have a long enough lens for the game drives. I was working with a 400mm lens most of the time and this was just about OK. You do not have the facility to use a tripod so bring a bean bag to rest the camera on the vehicle fittings. Oh, and charge your batteries at every available opportunity! I took about 16GB of memory cards, which I found sufficient to cover all the different locations we visited.