I had the same problem - 5ft 7 and for an unknown reason went from 9st 3 to 8 stone. I have got it back up to 8st 6 but have stayed at that for a few years, can't get any more on. I started by just adding a bit to each meal - an extra piece of toast or an extra scoop of porridge oats, a two egg omelette instead of one, a little more pasta or rice than normal. Try and add an extra 200 calories a day. I also treated myself to two squares of nice dark choc every day as I don't usually eat any sweets.
Please buy the swimming costume - swimming is great for building up muscles. (Complan, in my view, is disgusting stuff)