Driver's seat belt locks out when cabin warm

1st post - apologies I cannot see a thread on this already. Have a 2012 Roomster and the driver's seat belt constantly locks out in warm weather. Have currently reverted to taking the panel off and manually feeding the belt to get myself strapped in but not exactly safe, especially when trying to look out from a junction and a right pain in the a$$ to do. 

Anyone else experienced this and a solution other than a whole new mechanism? I have wound all way out and played about but no dice. Doesn't do it when a cool day and didn't fail MoT for it which suggests it was working OK at time of MoT. 

Cheers!