There are several explanatory comments on Indian currency in various parts of the forum. However if you are going to Kerala and relying on changing Sterling notes make sure you cash sufficient at the Crowne Plaza on the first day to see you through till the Thursday lunchtime. Including sufficient for tips to the driver, bus boy and local guide.
The hotel Poetree Saravar Portico, the houseboats and the Abad Whispering Palms had few rupees so exchanging Sterling was virtually impossible.
For those using ATM's there is one in the grounds of the Crowne Plaza, another at the bus pickup point in Thakkady and one outside the Uday Samudra in Kovalam. It is difficult for the bus to stop during the journeys.
Many in our group struggled at times to get rupees.