I had a can of this on the shelf. Phoned Halfords customer services and they gave me a phone number for the manufacturers. The manufacturers told me that my tin was one of the dodgy ones and to carefully remove it from my kitchen table, where I had put it whilst phoning them. The company are sending out a hazardous waste company to collect it and told me to put it somewhere where if it went off it wouldn't hurt anyone or make too much mess.
It's now in the coal bunker with bubble wrap around it.
It was more than half used but they are sending me a brand new full tin so every cloud has a silver lining doesn't it.
Two gents from the manufacturer of the primer turned up at my house today to pick up the used tin. They told me it is all because one tin in the batch exploded when someone picked it up in the shop but as a result they are trying to call in all of the tins in the batch.
Big bonus - they gave me four brand new tins in exchange so I'm OK for weld through primer for the next 10 years!