custody of tower keys Oliver,
As has been said the bells including any keys are the property of The Church and the incumbent is responsible for them to the Bishop and the Diocese, the Church Wardens if they exist are given that responsibility to report and maintain. They jointly and severally have responsibility e.g. if the ropes are used by someone to commit suicide by hanging.
Again changes though now only ‘major’ changes to the fabric require a faculty
If a Tower Captain (not necessarily a Ringing master) takes on that responsibility ...
We are all aware of the Revd Ellacombe. He could not have imagined that his ‘machine’ would decades later enable rings of bells to at least be chimed when they are no longer in a fit state to be 'rung'!
The only other comment I would make is that Bell and Ringing chamber and Tower door locks are often old to very old and the keys to old locks are often badly worn so care should be taken if it is contemplated to cut/make copies of these keys; using e.g. two keys with different degrees of wear can/may/will over time wear ultimately to destruction the internals of said locks