hi its been this way since ive owned it (3 weeks) the guy selling it said it had been this way a while, so basically you can lock the drivers door with key but it wont unlock with the key… a key is all i have by the way! so im leaving it unlocked at the moment, anyone know of this fault and is it fixable or do i need a new barrel?? the barrels not been damaged by theifs or anything either,
cheers guys 
Just sounds like something is not fitted correct in the door and making the lock jam.
Take the door card off and have a nosey.
M-m
Sounds like the small spring on the door lock has gone. This is exactly what happened to my car. There is a ‘how to’ on the forum somewhere explaining how to replace the spring. It’s a bit of a pig of a job but manageable if you have some patience. The hard part will be getting into the car in the first place. Contrary to expectation the door card does come off if the door is closed, you just have to wiggle it around (there are instructions on how to do that on one of the threads as well).
Just search for ‘door lock spring’ and I’m sure it will all become clear.
This should help…
http://www.mx5oc.co.uk/forum/forums/t/8246.aspx?PageIndex=1
I don’t think it is the lock spring as it works one way but not the other ?.
M-m
Actually M-M that’s exactly what happened with mine when the spring went. I could lock it with the key but not unlock it. (I verified this after I had got the door card off when I was trying to work out what was wrong and before I realised it was the spring.) I presume that the way the mechanism is designed the spring isn’t actually needed to allow the locking part to work but rather to re-position things so the unlocking part works.
I agree it’s not 100% guaranteed that it’s the spring but what he is describing is exactly what happened on my Roadster and it was the spring.
If the spring is broken/missing it will lock but not unlock. Paggers is correct, the spring returns the mechanism to the correct position.
M-m
I had a similar problem, but the other way round. I would have to lock the car by pushing the lever and while closing the door pulling the handle. After a few days, the issue just went away. It must have been dirt or something in the lock. I have fixed it “proper” by installing remote locking.
Cheers everyone il give the spring idea a go…any idea where i can buy one?? Thanks
Autolink have them buddy, get two for when the other side does not want to play nice 
M-m
Ordered see what happens hope its not to difficult…any instructions on here??
There is a ‘How To’ and a couple of threads with useful suggestions also (such as how to get the door card off while the door is still closed). Try the link below and you will be well on your way.
http://www.mx5oc.co.uk/forum/search/SearchResults.aspx?q=door+lock+spring