Thanks for the replies, the relevent ones at least
Sorry if I didn't make it clear but I'm not after a single do-everything lens, just advice on what my next 2 lenses should be, the first for longer focal length and faster focusing than my 55-250 and the second for better quality portraits / landscapes than my 18-55 and possibly with macro.
I understand that the Bigma is a great lens for image quality and reach but the lack of image stabilisation would be a big drawback for me as I dont tend to shoot with a tripod. I'm leaning towards the Sigma 150-500 as a compromise between price / performance / image quality.
I probably wont be able to look at getting the second lens for a while so maybe this would be better addressed when I can afford it as new options might have been released by then.