My one year old lab has always been of a slight build (she is a working type too) and over the last month she has been doing a lot more swimming and running. This swimming and running will continue as she loves it and I love watching her enjoy herself but she has lost even more weight.
Her back legs have bulked up massively with muscle and across her chest and shoulders but I can start to see her hip bones and her spine and ribs are becoming more prominant. She is already being fed almost double the amount on the back of the food bag and its a good quality food too. What can I do to help her gain a bit of weight so as not to see these bones?

You can either up her food or add extra protein to what she is eating. My boys work off their food due to exercise that I changed foods to give them more protein and carbs.

Try adding in fresh meats; I have one dog this way, same thing, there's only so much dry food you can feed them.

You don't want to add carb's, you need protein/fat....have you thought about feeding some raw meaty bones?

I feed Rivers Stella & Chewey's raw duck/turkey steaks along with his dry food, it's the only thing that's worked.

Don't know what is available for foods...across the pond...but they must have a performance food....usually in the 30 % protein....20 % fat range...this allows smaller portions to be fed and also keeps weight on very active dogs.

Not sure how easy it is for you in Dorset to get quality food from local pet stores, but I know I was finding it difficult in my area of the UK - so I get my pups food online using a company called zooplus. They sell both Orijen and Acana dry foods, which are really, really good foods. Acana Grasslands for example has 33% protein and 17% fat, while Orijen has even more protein, I think around 40%. Zooplus also have loads of other foods and definately have many brands who have a performance variety.

Also, as others have suggested, adding fresh meats, raw meaty bones could definately help.

Good luck and definately keep your dog swimming and running - I'm sure she's loving it, and a happy, tired out dog makes a good dog :)