Paul JS:
Unfortunately if you search consumer forums you will find many occurences of this.
The basic "rule" is that the process should not take more than 30 days starting from when the manager holding your original account receives the paperwork from the new provider.
What you should have done after 30 days had elapsed, and should now do as soon as possible, is to raise a formal complaint with Barclays strictly in accordance with the Barclay's complaints procedure. If you remain disatisfied after deadlock or after 8 weeks, whichever occurs first, refer the issue to the Financial Ombudsman Service. This is a ideal type of issue to be adjudicated becasue there is such a clear "balance of probability" that a cheque sent twice by courier is unlikely to have got lost twice except at Barclays. While the FOS process unfortunately may make up to year, if deserved a compensation award will be made. The process costs you nothing but Barclays have to pay a hefty case fee regardless of the outcome.