In most cases it doesn’t actually mean the salary is negotiable. You’re not going for the Chairman of ICI or similar.
It means they either pay X but its so crap they don’t want to display it. Or new drivers get X and slightly more experienced drivers get a little more (very slightly more). I suspect the former is more likely.
Oh if its agency, it means X for PAYE and another quid for LTD as its much cheaper for them.
As for numbers class 2 £9-10, class 1 £10-12, nights £11-13. Some get more but realistically even around East Midlands for general haulage thats about normal.