Hi Steve,
I'd agree with the advice given by D, but it might be worth checking
something else.
You mentioned that you'd cancelled the contract. You should know that
both accounts need to be open for the port to go through. So if your
account is scheduled to cancel on April 30th and the port is due on May
the 2nd, then the port won't go through. This is because the
cancellation would happen first and the port wouldn't be able to
proceed because the number wouldn't be active any longer.
I'd just check with Vodafone, via the webform at
www.vodafone.co.uk/contactus, that your account will cancel
automatically when the port goes through, rather than before.