In response to my enquiry, I received the following from Paul Jeffery, Assistant Curator of Geological Collections at the Oxford University Museum of Natural History:
‘Your find appears from the photograph to be a partial premolar of a horse (Equus sp.). It’s very difficult to give a likely age for a find such as this without either detailed contextual information, or radiocarbon dating, as they are present in many interglacial deposits from around 100,000kA right-up until the start of the Neolithic, and then are even more common in younger archaeological contexts. The appearance of the tooth depends more on the kind of material it’s buried in than its antiquity, so is no guide to age.
The balance of probability is that your find is 100s rather than 1000s of years old’.
This means, among other things, that Kat is holding the tooth upside down :).