Thanks for all your advice-not only about the lock but also for the computer stuff!!!
JohnD, my yale lock is like the first picture you posted and similar in size albeit somewhat older
The storm dooors are not the primary doors-there is another internal glass door but your point re security is a good one-is there a "better" lock to choose that would prove more secure-do chubb do a yale style lock ?
yes, if you are looking for a rim nightlatch (a straight replacement for your old lock, should fit into the same hole if you use the same backset) most of the better makers do one with an internal cylinder (so you can lock it and it can then not be opened with the inside knob without a key) and deadlocking.
Yale and Legge are popular and make quite a variety, Chubb do some but are expensive, especially if you get the one with a much more secure key (looks like a round bar with little lumps sticking out of the side of it). there is a technique now widely known for opening cylinder locks which have a basically flat key like a yale. This technique does not work on keys that are basically like round bars with notches or ears on them (the ones I know are Abloy and Chubb AVA system). Also I understand that most burgars do not bother with tricking locks open as they can usually force a door or window. For less money you could buy a British Standard mortice deadlock, and use this to supplement the nightlatch. If you go to a locksmith instead of a DIY shed you can get two (or more) locks suited to the same key, this reduces the number of keys you have to carry and makes it slightly more likely that people will be bothered to use them (I have my back doors and garage door suited like that).
Of the cylinder nightatches, the ones with a British Standard will generally be the strongest, but most will still have flat keys.
In his earlier post Big All posted a link to a supplier that seems to have a good range at good prices.
You could get a restricted profile cylinder, which makes it very difficult to get hold of key blanks to perform the opening method indicated. EVVA are a good range for such profiles, but there are many more on the market.
Best bet is to have a chat with a local locksmith, and mention you want supply only.
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