So ...
The lovely @Chimike welcomed me to his Aladin's cave of most things bass yesterday and kindly shared his skill in trying to resolve the issue.
After a listen, he described the sound as 'chuffing' rather than a rattle. Dissapeared into the treasure trove and when he emerged he popped 6 screws into the holes in the C4's back panel (Where the skids had lived).
No more chuffing!
Playing my bass/head, my bass his head, his bass his head ... all sounded OK, tho there was some power tool noise from adjacent workshops and we were outdoors (To avoid any resonance from all the gear he has ... it seemed to go on for miles, but in reality was probably just kilometers) so hard to guage, but a good sign.
I arrived home to late yesterday too try it out, but late afternoon when I got home from work I hastened to get my rig setup ... and it sounds fab!
A very happy camper!
Thank you all for your suggestions.
By the way, the seller (Top man and why BC is THE place to buy) totally lived up to his glowing feedback, despite not having experienced the chuffing himself he had offered to either refund me or cover the cost repair. @Chimike refused to let me pay for the screws (Or his expertise). I'm delighted that neither were required.
I've fallen in love with the my dimunitive C4 and, whilst it is heavier than I had aimed for in a cab, a hand cart will easily sort that!
S'manth x