According to the baa.com website there is but it ain't cheap. Make sure you email them to check where it is and that they haven't closed it since they posted this on the website.

Other option would be to see if you can get your luggage checked straight through.

Left Luggage & Baggage Storage - Operated by Excess Baggage Company
Each terminal has a left luggage facility for storing baggage for a number of hours or days. Items are security checked.
Charges: £5.50 per item per day (or part thereof).
Tel: +44 (0)20 8759 3344; email
Email [email protected].

Opening Hours:
Terminal 1 (06:00 to 23:00)
Terminal 2 (06:00 to 22:30)
Terminal 3 (05:30 to 22:30)
Terminal 4 (05:30 to 23:00)