My garden is an escape route for my next door neighbour and an estate agents.

However I was recently burgled by people coming into my back garden.

Can I put a combination lock on the gate and let my neighbour and estate agent know the code?

No it would not be acceptable you could fit a lock that requires a key to enter from the outside and a level handle from the inside.

Do you have a legal document that requires you to provide this means of escape, like a wayleave agreement.