I think that more hay will provide the actual weight gain rather than more hard feed. Although yes boiled barley is a good way to put on weight but without extra hay it probably wont work.

Have you actually worked out how much (in weight) hay you are feeding by any chance. It could be that with a horse that is not a particularly good 'doer' you need to feed more bulk feed (Hay) than you would to other horses. if they are under weight you need to feed even more until they are back up to the proper weight too.

One thing I would say is that although moment of madness mentioned feeding less haylage maybe that meant the bulkiness of it rather than the weight. Haylage as momentofmadness said, can made some a bit fizzy - although not in the same way that horsehage does - that is a totally different product. Horsehage must not be fed at the same quantity as hay.
HOWEVER, it is really important to remember that haylage is very very differnt to horsehage.

Haylage actually contains more water than hay does -i.e. it is moist and therefore has water in it. Hay has been dried and is lighter.

That means that weight for weight you get less haylage in a net than you would in a net of hay. That means you have to feed MORE haylage than you would hay.

Hay or haylage is the thing to put weight on - not more hard feed. Hard feed is like giving chocolate for energy. Hard feed is the energy food, hay is the bulk food for weight.

I dont know what the regime is - sounds like out all day and in at night. sounds like the grass is not enough - it needs replacing with hay (lots of people have been feeding hay this summer owing to no water and no grass growing). I think it would be good to work out what the horse weighs right now and then weigh the hay and see if you can give it a whole lot more hay. Remember that it is HAY which is the weight gainer every time, not the hard feed.

i would suggest Alpha A Oil for weight and condition, along with sugarbeet and build up mix. this is the time you want them a bit chubby ready for winter so get as much on her as you can! and if things don't improve then baileys no. 3 stud cubes will do the trick even if she's not what they are intended for!

Well now she is on the bran added into her feed and also more hay, been putting in the field too, She is looking a lot better!!
She is a bit odd in the way of what works for other horses generally dont work for her haha
I have started feeding more hay at night which does seem to be helping. I think it looks worse because she still isnt up to weight muscle wise as she had 2 years off but her coat condition is looking great again and she is fattening up a bit!!
Strangely enough she is already getting a winter coat! Think hse is preparing herself!! lol

That's good then. don't forget that bran causes a calcium deficiency if fed regularly so you might need to think about adding something like ground limestone flour. Bran doesn't have a lot of nutritional value at all either and acts as a laxative too - so watch out for that !!!

That cereal meal you / someone mentioned (Bailiey's No 1) is really really good to add to feeds for weight gain. I have used it on an older pony which needed to work a bit harder but found it difficult to keep on weight, but in case anyone else is thinking of using it, perhaps I should add that it MUST be fed as part of a diet not as a complete feed because it doesn't have sufficient vits and mins and etc.so isn't at alla balanced diet. It is ideal for weight gain where you don't want to add energy giving feeds.

I know you have to take care with adding feeds which give energy when you want to put on weight, especially to thoroubreds or thoroughbred types. It is easy to give them more energy (than they need) and then they just burn off the extra weight anyway (often by being silly and spooky ) so it is a waste of money in the end, in fact it becomes a bit of a vicious circle really. Give her as much hay as you can at the moment so that she has a good build up of body fat before the winter.
She will grow a coat early because she is cold - having a layer of fat keeps them warm so she is obviously feeling a little cold and therefore growing that winter coat early. Get the weight on quick now I would say. She sounds as though she has a really nice caring owner though so she is a lucky girl.

I just had another thought. Did you have her teeth checked ! - if not it might be a good idea to get an Equine Dental Technician in - I would suggest a UK qualified one though. It could be that although she is eating food well, she might not be chewing it properly (if the teeth have worn unevenly and so not working as well as they should) and therefore it might not be being digested properly so her body isnt getting the goodness out of it !

Well I had thought of getting her teeth checked but she really is eating absolutely fine and shuvells her feed down!! I water it down alot aswell. She is on a balanced diet I think as her feed is consistant and also I try not to feed her anything which is too heated. She has sugar beet added in the winter too.
She seems to be getting alot better very quickly so I am pleased! I think it does make it look worse than it is as she isnt completely muscled up to how she should be just yet but u can barely see ribs anymore which is a very good start!! Lol
I added some pics of us on our charity ride last Sunday in the photo galleries section so have a look and tell me what you think lol

Thanks for all the tips

Hay is the weight gainer - more hay and more fibre feeds. Try not to increase on the hard food - that provides more energy and that means more loss of weight.

Always use Hay (or haylage etc) as the first and formost thing to put on weight.

Extra Hay, More Hay and More Hay is what you should ALWAYS use first.