Guys, thanks for the advice and suggestions. Farnel seem to stock FM24C512 and I guess at £12 it's not bad when you consider the read/writes are claimed to be in excess of 10 billion !!

I'll also look at dropping the sample rate down, but when the room is cold and a heater should fail, the temperature in the reptile enclosures can drop quite a bit in a several minutes. IF the sample rate was a lot less, any graph from the resulting data would be a bit basic