It sounds very much like our 2008 Roomster, in the end last summer I replaced that lock with a brand new unit that I imported from Slovakia for around €120, I was quoted by a local dealer £220 for just the lock. Second hand parts are not that cheap considering they are likely to show the same issue at some point, although in hindsight second hand would have been a better option as the car was written off in February.
Some eBay sellers will list a Fabia lock as being suitable for the Roomster – it is not, although the main part of the lock is the same there is a difference in the latch. I guess it would be possible to put new internals into the old lock.