DON'T do what I did and search 'locksmith Harringay' and pay through the nose for some middleman service
Instead, call Mas Ali, The Lock Specialist, direct on 07551916691, he was very quick and very nice.
I recommend saving his number to your phone, as I have done, so if you get caught out like I did you have help at your fingertips.
... or bury a spare key in your garden. Grease it first, wrap it in plastic and put in one of those plastic pots that pills come in, or else when you need it in five years' time it will have rusted enough to snap in the lock.
Thanks for the number though, there are other reasons to need locksmiths - like if your bag is stolen with your keys in.