Buddy training once a week is not going to do anything for your lose skin I am knew to this board but if there is one thing that i can give advice on is nutrition and exercise. So when you say lose skin I am taking it as the only place that you have it is around the chest.
Ok well lets take a look at the other factor you are down to 13 stone from 16 this means that there is a lot of fat lost aswell. So what you really need to do is try and bulk those muscle up get the muscle as big as they can I know you are worried about you overall weight but the things is the more muscle you have the more callories it takes to maintain so therefore the more muscle the more fat you will lose and faster. But by building the muscle in the upper body you are going to resolve most of the lose skin.
So the next thing is what training to do well in most cases to build muscle you will have stop doing cardio concentrate on weights only and prefferably free weights. And a different muscle group each day so lets say
Monday arms Bicep,Tricep
Tuesday Chest
Wed Back
THurs Shoulders
Fri Legs
I know that you are not worried about your legs but you do not want a massive upper body and no legs to show off.
The next thing is nutrition I will really need to know what you way in pounds or kilos to e able to help you out here I will be able to work out the calories that you need to build muscle and what foods you are going to need. But as a start you need 40% protein 40% carbs 20% fat that is general rule.
Really easy actually I will give you a easy way of counting calories.
Protein 1gram of protein has 4 calories
Carbs 1 gram of carbs has 4 calories
Fat 1 gram of fat has 9 calories
You need to consume enough of these food sources each day to supply your body with energy but also supply your muscles with the nutrition they need to grow.
When it comes to these food souces it has to be clean foods protein things like egg, Chicken, Steack the list goes on. Carbs stay away from sugars and eat clean carbs things like brown rice, pasta, bread. Last but not least fat you need good fats things like omega 3, olive oil once again the list goes on check out this site WWW.BODYFORLIFE.COM it was a great souce of information when I first started to train. There is a full list of foods that are good for building muscle. Please come back with your weight in kilos and I will tell you your amount of calories that you will need to build the muscle and get rid og the unwanted lose skin. And also any other questions that you might have.