How very odd. Did you quote the GRO reference number when ordering?
Anyway, you are in luck, as the relevant register office is part of the Cheshire BMD project. Go to their website at
www.cheshirebmd.org.uk and search for the marriage then follow the instructions to order a copy from the local registrar. You will have to do this by post, unfortunately, at it is a wee bit more expensive (£10.00 rather than £9.25).
Caroline