Believe it or not after heating the mould that was my next step, but, even though I have a huge oven the only clamps I have that can exert enough pressure are too big to fit in there. See what happens with the next mould I make, I have an idea that might just work. The problem with HDPE is that it warps and shrinks as it cools. The good thing about HDPE is that it is always recyclable so even my cock-ups are not a total loss.
TC