For the record, Terminal 2 it is to the left of Lufthansa check-in downstairs.
The one in terminal 1 is quite well hidden, behind some shops an downstairs, as I remember. It is signposted though.
In fact, go to baa.co.uk, put 'left luggage' in the search facility and you'll get the details for LHR...as well as all the other BAA Airports.
WT