On the rusty tank it is usually better to add a couple of handfuls of gravel or drywall screws and then shake this about. It will knock off all the flaky lumps and then a rust remover or acid will strip the thinner remaining coating of rust. I recently bought 1kg of citric acid on Ebay and it only cost about £6 including p+p. It works well but takes quite a lot of time. 1kg is good for 10 litres of water. If you want HCl then you will find it under its old name, Muriatic Acid. You can find it at all the DIY stores and it is what's sold for cleaning mortar off brick work. Will you leave the inside of the tank as bare metal? Do you have any experience with internal tank coatings that are available in the UK? I am interested in this because I have a tank that needs cleaning, similar condition to yours. Thank you.