Hello Barry,
I was not familiar with the Gas Trac product, but I have checked the Calor web page for the product, I note it is designed for their range of BBQ/Patio gas bottles, but the way you have posted made me think you may have somehow fitted it to your caravan.
However the page clearly states:-
"works much like the petrol gauge on a car. The indicator shows 'hi' when the gas cylinder is more than half full and moves to 'lo' when you're running low on gas"
This implies it actually senses the top surface of the liquefied gas and moves according to where the level is. Though I find it hard to see how any mechanical float and linkage could be fitted in the space available to achieve this, it looks more like a pressure sensitive gauge, which is unlikely to give reliable results until the bottle has used up al the liquefied gas and only has vapour left in it
Your experience with the product shows it did not work.
If it was used on a BBQ/patio heater then it has not performed the job it was designed/intended or sold for. That is a breach of the SoGA, and even if the manufactures guarantee has expired, I think you may have a case under the sellers liability "as not fit for purpose"
However, If you have some how managed to get it fitted to a caravan system, then, you may not be using it for its designed purpose, and whilst I agree the difference may seem to be academic, it is enough for a SoGA claim to be denied.