Ask them to pay the standard rate of interest (about 4.5% i think currently) on the amount owing from one month from the date the amount was owed. (That sentence reads like some bank blurb...
). So if they owe you £200 that's approx £20 more over the two months from August 14 to now. Otherwise threaten with legal action...always works for me. No one should tolerate waiting this long to be paid. Takes the P!ss.