Sand Martins look very similar to House Martins. The main difference is colour - House Martins appear deep blue, and Sand Martins brown. However when they are flying overhead this will be very difficult to see. A better way to distinguish them is the House Martin has white all over its underside, whereas the Sand Martin has a very clear 'collar', which you can see in the photo below.

Most of the time you will just see House Martins at Croome. I was surprised to see this Sand Martin in with the House Martins flying over the house and lawns (there may have been others with it). Sand Martins will not nest at Croome, they nest in sand banks, possibly in nearby quarries, so this one was just here for the food.