Epsom salts

Yes, the difference is that sold by a chemist will (almost certainly) be made under stricter conditions than 'garden' grade as it is intended for human use and likely labelled as 'pure'. Don't get the 'fizzer' bath soak stuff though as that contains sodium bicarb, citric acid and various other foam stabilising chemicals.

Sulphate (Epson Salts)   Is this the same they sell in a chemist from a garden centre  Will this be okay if I bought this from a chemist for my allotment

Is this one okay for my plants bought it from a chemist

This is John's article about using Epsom Salts for yellowing tomatoes.

https://www.allotment-garden.org/vegetable/how-to-grow-your-own-tomatoes/tomato-troubles-diseases-causes-cures/tomatoes-magnesium-deficiency/