Anybody know much about Birdy folding bikes, in particular the deraillieur and chain-ring set up?

[Cyclopathic]

I've had one in for repairs and it is proving to be a pain. I have not been able to solve the riders problem of the chain coming off. The point of detachment seems to be the chain-ring. I have already had to remove, straighten and replace the cage that attaches to the rear mech to stop the chain getting tangled up when the bike is folded and everything to do with the rear mech and cassette look to be working just fine.
It's almost as if the chain were not under enough tension but the spring on the rear mech seems good and strong.
Anybody know what the problem might be and if it's something common to Birdy or not?