I have good recent experience with a leak detection system that uses a special fan to pressurize the interior, and they search for places where the positive pressure inside is pushing out through seam, joints and seals.
RV leak detection is offered by numerous providers in the US, but I don't know if there are mobile caravan leak detection services in/near Hertfordshire.
The machine they used for mine is called a Sealtech 430r. Sealtech has a list
of some providers who use their machine, but they all seem to be in the US and Canada.