iMessage activation problem - wrong number

Hi all, hope you can help.

I'm using an iPhone 5 with the latest version of iOS.

I switched from Orange (UK) to EE (UK), which meant I got a new nano SIM for my phone. I've put that in, and the switch in networks has happened overnight.

However, iMessage isn't syncing up or working properly. I've toggled iMessage from On > Off several times, but it's constantly saying 'Waiting for activation...'.

When I look under the 'Send & Receive' menu, all of my iCloud emails (which I also use for iMessage on iPad etc) are there but my phone number is wrong. It's also greyed out, so I can't edit it. It looks like the number showing there is the one associated with the new SIM card I received - despite my 'proper' number being ported across.

Don't know what to do. How can I fix this issue?

Already I've tried: Signing out of iCloud, signing back in, making sure my correct number is displayed under 'Phone' (it's correct there), taking the SIM card out and putting it back in, switched the phone on and off several times, done a hard reset (top button + home button). Nothing worked.

I'm out of ideas. Can anybody assist please?