All now seems well. The caravan has been inspected and needs a new rear bumper panel. The tow hitch tested and no fault found. Conclusion: I must have been at fault in hitching up. The guy who did the inspection said that the new AL-KO hitches have the red/green indicator obscured by the stabiliser bar so its possible to think the van is hitched ok when its not.
Thanks to all those who responded - lots of useful info which others may find useful.
The moral of the tale is: Check and check again before moving off, or buy a campervan.