From what I can make out, it seems like the glitch was in the credit card you used for the transaction, many times a CANADIAN bank will not allow you to use your credit card in another country as they feel this could be a fradulant transaction, UNLESS you have notfied your credit card bank about the TRIP you are making, then they remove the flag and allow you to use the credit card in another country.
When I took a trip India in mid Feb I had to explicityly notify my bank about this and they said now you are safe to use your credit card in India.
They do this to save us from someone else using your card in another country.
Because the part you mentioned about UK card working at the same time leads me to belive it was your credit card being the culprit.
Take care,
DNT2003
Hi, further to the above, we tried some more times with our Canadian visa card and got nowhere. We do not yet know if they took the money out but will monitor carefully over the next few days.
However I then tried with our UK American Express card (which we didn't want to do as we have to pay a foreign transaction fee) it worked out ok and went through.

So I guess the message is, if payment doesn't work first time, try a different card!
Failing that, we would have had to get them to send the form to a relative in canada to pay on our behalf and then mail the receipt to us, which probably would have taken another few weeks. Thankfully this wasn't necessary.
Still, CIC - sort it out! Not good enough in this technology era to have silly problems like this!