Normally VAT refund should be processed within 4 to 6 weeks from the time you posted the stamped tax refund form. If you haven't received money after 6 weeks, you should contact the the VAT refund company, which is displayed in your receipt or tax refund form, and you will also need your voucher number. So it is always a good idea to photocopy the receipts and refund forms before posting them.
Below are some contacts for the major Tax free agents in the UK:
Tax Free Worldwide UK Ltd :
Tel: +44 207 612 1560
Website: www.taxfreeworldwide.com/uk

Premier Tax Free UK Ltd:
Tel: 01737 644922
Website: www.premiertaxfree.com/

Global Blue:
website: www.global-blue.com/