So, it's not just me then.
I've begun to notice that the Humbrol lids won't stay tight on the tinlet, that includes after turning upside down for a few seconds and then righting.
It's almost as if there is sufficient out-gassing pressure from the paint to push the lid off. I've taken to flipping the tinlet over to get paint to form a seal, then putting the tinlet down with a metal weight on top of the lid and checking it every couple of minutes until the lid stays down. Judging by the amount of paint/carrier that later ends up above the lid it's pretty clear that it still isn't a good seal.
I began to wonder if they'd redesigned the tinlet/lid so that once opened the lid or tinlet rim got distorted.
Would be interested to hear others experiences.