I'm not sure what the age is, but it is worth noting it relates to the calbriation check of the meter, rather than the age of the instrument itself, meters could be swwapped out for a replacement, the old one sent back to the workshop, calibrated ans used to swap out someone elses meter, now in the days of electronic and smart meters, I'm not sure this happens much, but it can.
As to the isolation for your meter, chances are what would happen is the meter fixer would kick it back, reporting no access to isolate (they have a system to report different issues to the DNO, each with a number). The DNO would then have to look at dealing with the problem, that might involve a red isolation link next to your meter and as to how that gets fitted, depending on how its risk assessed, the DNO are allowed to work live in a lot of instances, or if they deem that they can't do that (too close to earthed metal parts, etc) then they do have a right of access to maintain their equipment, thats usually enough for most stubborn people to relent, if not then warrents can be obtained and they can enter with assistance of locksmiths and the police if required (thats they are sometimes required to do if they need to remove someones fuse becuase they refuse to pay the bill!)
Things will happen slowly if for a meter change, if the fuse blows and needs replacement, things would end up happening a lot faster!
Why is the chap being obstructive? Does he own the flat? or is it rented?/ HA? / Council? I know that when doing work on communial supplies located in one of the flats on council blocks, most of the obstructive folk are generally more receptive once the housing department spell out the fact that they'll not win by being obstructive....
