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Has it failed open or locked closed?
Wether it's open or closed, a couple of these (top and bottom) of the door are a good way of adding extra security for wood, pvc and aluminium framed doors. A locksmith would likely have to order parts for the lock, so probably end up fitting these in the meantime.
https://www.screwfix.com/p/era-100-12-patio-push-lock-white/2430t
If it's open and the sliding leaf is on the inside, wedging a length of broom handle to prevent it from sliding is a quick temporary measure.
Question for the hive mind - can anyone recommend a locksmith in SE London? Someone with experience of sliding glass doors preferably. Our latch mechanism has failed and need some help.