Hi ..that;s right..It's a single chain ring on the crank set..with no front derailleur.
The chain just keeps missing the teeth in highest rear gear..so I adjusted the rear derailleur to avoid the highest gear, as a stop gap measure, but when my friend attempted to ride it..off it came again.
Crank set looks like it could accommodate an inner chain ring so thought I might swap the chain ring to the inner position to move it towards the frame 5mm or so and thereby straighten up the chain angle from front to back..but it's odd that it's doing it in the first place.