Dave Hutton
Warwickshire Birder
I am a member of the Birmingham and the Black Country Wildlife Trust and have a card which entitles you to visit almost all the WT reserves in the country except the ones in Norfolk, however I've not used it yet and don't know about Whitacre Heath's combination lock. Perhaps you have to get in touch with the Warks Trust or check out their website.
You will probably find that the combi lock is broken & that the lock is only closed to or the chain is wrapped around the post on closer inspection??