AWimbaWay. Sorry for delayed reply - I did add a message when you asked about this on the Christmas bargains thread but didn't check here until later. I also had problems with the code last weekend and emailed them (well, filled out the form on the website) and they wrote back to say that it was a once only code. I then checked back over my orders and realised that I'd only ever used MUMSNET20 and not the new one so I wrote and told them. Also the message I got was invalid code rather than once only. I've now had a reply apologising and crediting me with the amount I would have saved (yet another excuse to spend more money!) It might be worth you emailing them and querying it as well.
Have heard this morning of a couple of other people struggling to get the code to work. FWIW I've used the old and new mn codes to work and they both have for me. Worth emailing them if other people are having the same prob as it sounds like a fault their end.
Which code are you using? I had trouble with the original code 'Mumsnet20' when trying to pay with paypal, but it worked fine when paying by debit card. Haven't tried the new '20MUMSNE15' one yet but could be the same?