custody of tower keys As David says, in law the bells are the church's bells and not the ringers' bells, so if the church authorities want to retain control of the keys, there's nothing in law that the ringers can do about it.
Having said which, if the ringers are expert in bell maintenance matters, to my mind it would be good practice for them to have both access to and effective control of the bell chamber (i.e. one key for the bell chamber, and that held by the ringers so the vicar can't just wander around bells that might be up). But given what the law says, this is something they must negotiate with the incumbent and wardens - they can't demand it.
And looking at the specific case here, apparently the bells 'don't get rung much'. In which case everyone would also need to consider whether there are ringers around who would be suitable and would want to hold the bell chamber key. It might be that 'the tower captain' that Oliver met is in fact the last remaining learner and has never got beyond ringing backstroke under supervision.