Thank you everyone. I went and let a bit of air our first thing this morning - oily calm so no sailing and came home again !
Happily I can now enjoy getting overheated in the garden without worrying about the dinghy ..... expect there will be another thing to worry about very soon ......
Thermal expansion is about 6% from 20 to 40 degrees celsius.
Most inflatables are never pumped up to the right pressure anyway, hence I am not too worried.
More damage is done when tenders are not enough inflated with seams folding, creases in the fabric when stored etc.