Hi there ​@Chris Wand 

Were you an existing customer whom upgraded with a new phone?

If so and the SIM holder is either the same size or smaller on the new phone then simply swapping the SIM into your new phone should be fine.

If you’re a new customer then it might be quicker to pop into Currys and ask them for a new SIM. They may even offer to register it, or failing that it can be done yourself.

edit. Without installing the app:

1. Visit this website: https://my.idmobile.co.uk/activate-sim-outside.
2. Tap ‘No, not yet’.
3. Choose your plan type.
4. Enter your iD Mobile number.
5. Select a contact method to receive your security code.
6. Enter your security code.
7. Tap ‘Submit security code’.
8. Enter and confirm your 19-digit SIM card number.
9. Tap ‘Activate your SIM’.