Nigel, you're talking complete nonsense!
The tank is vented, if it wasn't you couldn't fill it with water. Water enters and air escapes, you replace air with water- basic physics. Where do you get the idea that condensation can create pressure? For one thing, once the tank is filled, there would be very little surface area available to condense on. For another, all that could possibly condense has had to originate from the same source to begin with. If the source of the pressure was internal then the tank could only expand, it wouldn't contract surely. To contract the pressure would have to be applied externally. To contract internally, you would have to create vacuum at a greater rate than the vent can counter balance at- and nothing we are discussing here is going to do that.
My guess is the sides of the tank are expanding with the heat thus pulling the top downwards- or the top of the tank is weaker than normal and just sinks with the heat.