Thanks Richard - sounds good advice and pretty much the same procedure as I have used for a long time.
The tow ball is a detachable fitted by the manufacturers (Vauxhall 2013 Zafira tourer)and was checked as being suitable when we collected the new caravan. Before detaching I had towed the unit through a very tight right hand u turn through the access gate and then over a substantial speed hump. The incident occurred when I applied left lock up a very slight incline to join the main road. The second detachment happened at exactly the same spot which is why I think there may be a fault with the hitch.
Thanks again - the caravan is now to be inspected early next week.(just had a phone call from the sellers)