A very common fault, and strangely mostly on the drivers side.
A new door lock is about £150. No doubt double that from the main dealers, and go with a spare arm for them to rip one off.
A used door lock on eBay about £50.
Or potentially a new motor for about £10.
I collected tons of info recently.
Mx5 city have a new on on offer for £119, but expect carriage to London.