If someone refuses to pay you money that they owe you, all you can do is to threaten and, if necessary, take legal action against them.
Type out a (dated) letter, roughly as follows. Send it by recorded delivery and remember to keep a copy of it:
"Dear Mr Bloggs,
Further to my phone calls (to which I have received no satisfactory response), I write to remind you that I have not received payment in respect of my final week of employment with your company. (i.e. from Monday xx/yy/06 to Friday aa/bb/06). I calculate the payment due to be �cc.dd (less statutory deductions), based upon employment for ee hours at �ff.gg per hour.
I look forward to receipt of this payment in the near future but please note that, if I do not receive this payment within 14 days, it is my intention to pursue my claim for payment (together with associated court costs) through the civil courts.
Your faithfully,
R.C.L."
Chris