Hi Paul,
If you have any car DIY expertise, particularly on older generation cars like the Mini or similar, then it's not a difficult job. But if not, then I'd do as Mike suggested and get a mobile engineer to come to you.
But in outline, and working from the wheels forwards:
1. The brake shoes (inside the brake drums) might be worn - and need replacing
2. The brake shoes might just need adjusting - there should be adjusters on the back of the drums (just like the first generation Mini) that you click round to set them correctly.
What's correct? So that the drums revolve easily, with no dragging, but as soon as the handbrake is applied - or the over-run operates - then the brakes should work
3. The cables are out of adjustment where they meet the balance bar - this sets the side to side balance - careful with this adjustment
4. The handbrake lever is out of adjustment or has become disconnected from the mechanical over-run brake system
If you intend to progress further yourself, I'd suggest getting hold of a caravan servicing manual - like the Haynes Caravan Manual 3rd Edition - from Halfords, Amazon, or Haynes themselves - about