Voltage sensitive fridge relay faulty

is there not the option to tweak the relay? Some have a potentiometer on them to allow you to adjust the point at which they trigger

The fridge is cannot drain the car battery unless the caravan and car are connected and the car engine is not running.  How often is that the situation and for how long?

Whatever the situation you should charge the car battery.

Yes, there is. It was adjusted last summer in accordance with advice given. Now however if the battery has a resting voltage above 12.28 the number 10 stays live until the parasitic drain pulls the voltage down.

The caravan is not connected, I think that despite previously adjusting the VSR it must have gone off setting and I am considering replacing it with a factory set one with cut in at 13.5 and cut out at 12.8V.

but as LLM says, no matter what voltage it opens & closes, if there's nothing plugged into the socket, then I'm not sure how the relay is the cause of your battery being discharged. But can you not tweak the relay again so that it needs more than 12.7 volts before it closes ie when the engine is running

This what I am thinking, just take some time as most of the trim will have to come off in the rear inside quarter to get at it. 

Thanks to all for the comments and advice.

As you say ED an open circuit can't drain the battery. The trip voltage of the relay is just a red herring.  I wonder how long the car was sitting unused and unconnected prior to the OP finding the low battery voltage.  

2 days and it is garaged.

Whether it's garaged or not is most probably irrelevant.  I would be looking for a drain such as an internal lamp, perhaps the boot light, roof light or maybe the glove box lamp.  Failing that it needs to go to the agent for a check con their diagnostic kit.