One niggle and one cautionary note though: the quality of the lock body seems to be deteriorating; and the cost of a replacement cylinder sometimes causes people to adapt a cheaper cylinder and use it in place of a new Ingersoll cylinder.

It’s possible to come across old Ingersoll SC71s that are still working after 20 or more years. But these days the lock bodies seem to last less than 10. However, for those 10 years it’s still an excellent lock.

If you have an Ingersoll lock, then before congratulating yourself check the cylinder to ensure that someone hasn’t replaced it with an inferior one. The genuine Ingersoll cylinder is quite unique. It has a slightly domed front, the keyway is wave-shaped and symmetric, and the key is double sided in that, unlike, say, a Yale which would have teeth only down one edge, an Ingersoll key has teeth down both edges (and to help you insert it the right way round, the hole in the bow is offset).

Well you can be fairly sure that a BS lock is reasonably good. However, the reverse does not necessarily apply. Just because a lock isn’t BS, it doesn’t mean that it’s of lesser quality. Two of the best locks ever — the Ingersoll SC71 and the Chubb 110 have not been BS for much of their career. Yet if you have that pair of locks on your door, a skilled locksmith will have a very tough time getting you in, so a thief is in for an almost impossible time. Although as we’ve said many times, the locks on the front door are only secondary in importance to physical strength of the door and frame, the illumination, the state of the window locks and what’s around the back.

It doesn’t necessarily mean that your insurance company will be happy either. I’m afraid you really do have to read the policy. The commonest thing your insurance will ask for is a five-lever mortice deadlock on the final exit door (the one you can’t bolt from the inside because you will be on the outside). It might go further and ask for a BS 3621 five-lever mortice deadlock. It might just ask for a BS 3621 lock.

If you do have the excellent Chubb 110 and your insurance policy asks for a five-lever deadlock you ought to get written confirmation (good luck) that the Chubb 110 is acceptable (as it should be) since for technical reasons lost to the mists of time, Chubb decided to term the 110 a five-detainer lock. It is, as I’ve already mentioned, at least as good as, if not better than your typical five-lever lock.

Not all mortice deadlocks are lever locks. In the States, for example, very few locks are lever locks. The other main kind of lock is a pin tumbler lock. Your Yale latch lock will almost certainly be a pin tumbler lock. So what? Well if you squint into your pin tumbler lock’s keyway, you will actually be able to see the pins. They’re little brass pins of a couple of millmetres diameter. If you can see them though, you can attack them. The lever tumblers of a lever lock are deep inside the lock and way more difficult to attack.

It could turn out then, that you have a pin tumbler cylinder operated mortice deadlock on your door. If you have, its key will not be the traditional “keyhole” kind of key with a blade and a stem. Instead your key will be a “Yale” style key turning in a cylinder and with the cylinder operating the deadbolt. Again, if your insurance policy asks for a five-lever mortice deadlock, then you might not be covered. Get written confirmation.

If you have a pair of keyed-alike (same key operates both) Banham locks, which are quite common in London, and your insurance asks for a five-lever deadlock, once again you should get written confirmation that your locks are acceptable. Banham do produce a fine lever lock but the typical pair of Banham locks encountered on London doors will both be cylinder operated and neither, therefore, will be a five-lever lock.