If you need an easy source for Magnesium Sulfate (Epsom Salts) you can currently get it from Asda in the toiletries section. It's 'WESTLAB Pure Mineral Bathing Epsom Salt', back of the pack states 'Westlab only source the highest quality Epsom Salt, Pharmaceutical Grade Standard'.
Not a bad price @ £5 for 1kg. I know you can get it cheaper but there's no postage costs for this so it works out a slightly better deal and since it's the fert that runs out the quickest it's handy to know that there is an easy bulk option available locally if you run out.
I bought some to have some spare, out of curiosity I have opened the bag just to check its real and can confirm that it is indeed the real deal (going by crystal shape). Importantly it is unperfumed, I gave the bag a good sniff when I opened it and all I can smell is the bag plastic.