Cheers mate, heres what I have done.
Today I went to Wickes and picked up some 12x100mm Heavy Duty Fixers, turns out the brackets supplied plugs can't have been designed for a 1/2'' hole as the instructions say... I drilled the holes deeper and the wickes plugs fit quite snug. I then had to drill out the holes on the bracket by a few mm to fit these new screws through. Locked them down tight, two do not seem to tighten completely but the bracket seems to be more than sturdy enough, especially as I have 'upgraded' the supplied fixings by a substantial amount.
TV is up and looks great! Just going to keep a close eye on the bracket in the meantime just to be 100% sure