Hi hardworkingdude
This is one of those questions which was raised even before the first iteration of CDM came into force.
What you have in effect is a term contract with your parent company as the CDM Client with you likely acting as the Principal Designer and Principal Contractor and with one or two Contractors depending on whether you need to bring in the locksmith subbie or not.
Not really any different in concept to a term maintenance contractor doing repairs to a road network, sometimes at short notice.
So, the key is how the CDM "project" is defined.
If the Client decides that your contract with them for whatever length of time counts as the "project", then it should be relatively straightforward to comply with CDM without generating disproportionate paperwork.
On this basis the "project" will be notifiable, but you can notify a period of however long the Client chooses (and possibly update the notification should that period continue) and indicate the "site" as being at various locations, not all yet known. If you can give some broad indication of area, e.g. a county, then that would be helpful.
Perhaps there are very odd occasions when a particular work package might be notifiable if considered separately. If so, probably prudent to do an update of the notification for each such package.
Which then leaves in a position to have a broad statement of design strategy, and a Construction Phase Plan that covers the broad principles that will apply to the term contract - so as example, I guess you have a contingency plan for when those having meters changed against their will might resort to verbal or physical violence.
Then you can add to that overarching CPP with works specific information. What is different about places A, B and C from the norm?
Plus when work is complete at each location, then there is likely to be a need for information to go to both your parent company and the occupier as to any specific H&S considerations that have changed for the premises going forward - otherwise known as Health and Safety File, Operations & Maintenance Manual or whatever.