I recently bought some Parker cartridges for my lovely Duofold. The shop sold washable and non-washable. This may seem a very silly question but what is the difference? Is one just more permanent? As in - it won't come out of your clothes if you spill it?! Explanations gratefully received.

I believe you've already answered your own question. Washable can be washed of your clothes. I don't think either of them is permanent when you spill water on your writing. I've only used washable but I've read(so I'm not sure) that the non washable is a bit darker in colour.

I'm curious where you bought the cartridges. Around here (Southeastern Pennsylvania) , most big box stores like Staples or Office Max will have only washable blue or black cartridges and if I am very lucky, Black Quink.

Previous answers answered pretty well. Both washable and non-washable are no good at resisting water on paper, but the washable is more easily washed. And non-washable is darker.