Don't do it, Karen. Honestly. I put mine on Burns for a while and they looked TERRIBLE on it and lost a lot of weight. I would say 'yes, it works for some dogs', and I am sure it does, but I have seen in person and heard from MANY people online that Burns has had the same effect on their dogs. I don't know of any other food that so many have had issues with.
Not only that, but you can get better for your money. Burns has a very high grain content. In some of their formulas, amazingly, the grain content runs as high 70%. Considering that you are paying £40-45 for a 15kg sack that is mostly grain, it doesn't exactly represent good value for money. High grain also means a very low meat content.
I would recommend Skinners or Arden Grange. You will get far more for your money in terms of price and quality of ingredients. If your pet shop "cant'" (can't be bothered more like...) order in foods, frankly I would look elsewhere or order online.