Back door Lock will lock but not unlock - help

[blueseventytwo]

Hi
First time posting here so please be kind!

My back door lock will lock ok but not unlock from the inside. If I go outside, it unlocks ok. I have tried WD40 which is about the extent of my skills and hasn't done the trick. The door is about 10 years old. The lock type is ERA 5 Lever (Made in England).

Are there any simple tricks I could do to sort this out myself as I am a bit strapped for cash. Many thanks

[femurat]

The key may be worn or broken (missing a bit) making it work only on one side of the door.
The lock could be worn or loose... there're too many options to tell it without knowing more about your lock.

Are you mechanically inclined? If so you may try and disassemble the lock to check if there's something wrong with it. This is a high risk option: you may end up without the lock... or with the lock that no longer opens the door once closed...

I suggest you get a new lock and play with this one and learn how it works once is no longer in the door.
Or remove this one from the door and put it back only if you're sure you fixed it properly.

Cheers

[blueseventytwo]

Thank you femurat - really appreciate the help.... not v mechanically minded but at least I now know there's no real simple trick I'm missing that I could do myself

[mhole]

Two likely causes.
1.Worn or damaged key
2.worn curtain

if you put the key in, can you wiggle it around like a joystick? Can you see any obvious asymmetry to the key?