For a 10k? Really? You'd better be aiming for 30:00 - 35:00.
I'd have thought just drinking enough water and eating enough clean calories would be enough, unless you're cross training massively or a vegan.
I now use myprotein.com. You can find discount codes online easily enough, and if you can stomach the unflavoured, it's very cheap and doesn't have any junk in it. I go for the smooth chocolate: it doesn't contain any aspartame or anything. I pay about 38 pounds for 2.5kg, I think.
That's a very good price for the Hench you've found. I've seen reviews that have said the whey protein is 50% sugar, but perhaps they were getting confused with the mass gainer product.