I think they are mainly sold for commercial growers, ie in bulk. If you used good compost to start with but still want to add some you could try Miracle Grow. Some people recommend a high potash fertiliser for potatoes, so if you want to try that use tomato fertiliser. If you have more than one bag why not try one of each and compare the result?

Thanks for the advice, during the 'banking up' I have been adding Chempak granules, now I have the foliage I was looking for an easy feed, I have plenty of Miracle grow and tom'feed I just thought that some company would have came up with a bottle of liquid for potatoes. I will do as you say, THANKS

There is a specific tomato fertilizer but I didn't think it was liquid. Worth looking in the shops though just in case. I wouldn't have thought you needed any though if you've put all that fertiliser on already?