From baa.co.uk (Heathrow pages...)

Each terminal has a left baggage facility for storing luggage for a number of hours or days. Items are security checked. Left baggage offices also offer competitive unaccompanied baggage rates and services and sell luggage and travel accessories. Charges are £5.50 per item per day (or part thereof). The service is operated by the Excess Baggage Company, who can be contacted on +44 (0)20 8745 4599. Opening times are:

Terminal 1 (06:00-23:00)
Terminal 2 (06:00-22:30)
Terminal 3 (05:30-22:30)
Terminal 4 (05:30-23:00)

This facility used to be located on a mezzanine floor between intl. arrivals and all departures. From departures, access was at the back of Burger King, but this may have changed.