Advertising does not guarantee sales if you are pushing a product that nobody wants at an uncompetitive price. Also, saying that you spent £40 in 2 weeks is meaningless, for example if the item you are selling is £1000, then £40 is not unreasonable if a new product.

Not sure if there is a formula for how much you should be spending but I suggest you study the data provided in your advertising section and home in on improving your keywords and search terms.

Also, try pushing your product in other ways, eg blogs and discussion boards