Question Author
Thank you for your input folks.
Just in case you are interested, I lined up lots of different cleaning products, soft toothbrushes and cleaning sticks, then last minute chose to use just warm soapy water.
It was a mistake! Although the tin is now reasonably clean, some of the colour came off, and the picture faded. I have dried it thoroughly, and it has been left out to air.
Oh well. It could have been worse if I’d used any chemicals!
The tankard, however, cleaned up nicely.