Did you ever fix this? Locksmith Holmfirth
You can order a new era gearbox for £9 from trade suppliers.
Take around 20 mins to get mech out the door and replace.
You may also want to adjust the door to ensure its smooth operation as a stiff or unaligned door is usually the cause...
This blog explains how to repair them, the replacements are very expensive.
http://midlandlocks.angelfire.com/blog/index.blog/1427229/upvc-mech-replacement-parts-locksmith-blog/
This is the problem 99% of the time, just make sure you remember how it all goes back together.
locksmith...