Doesn't look to bad mate, i've seen worse so you've got a good shell to work with. The link for frost didn't work but i buy a few things from them and this is 1 of the products i use for removing rust http://www.frost.co.uk/item_Detail.asp?productID=8229

For really thick rust i spray it on a leave it awhile then attack it with a wire brush attachment on my grinder