As nobody seems to have answered Mr Red's:

Even today with the formula changed (and ownership) I have tried to clean the tin with hot water - closing the lid and shaking causes an build-up of pressure inside. Mostly burnt sugar - what is going on there?

Risking that he knows the the answer and is just dropping a bit of predigested bovine flavor bits on us -

When you put hot water in a can and put the lid on it, if there's enough air to allow you to shake it, the air usually is quite a bit cooler than the water. When you shake it, air and water are splattered in and amongst each and t'other so that they become the same temperature. The tiny loss in temp of the water (very hight specific heat relative to air) causes very little contraction of the water, but the warming of the air causes it to wish to expand, often quite a bit. Since it's contained in a fixed volume because you closed the lid, the pressure increases.

The burnt sugar has little to do with it. Hot water, shaken in a sealed can with initially even slightly less hot air, will always show an increase in pressure (sometimes sufficient to make getting the lid off immediately a bit risky).

John