Hey Callum,
That is very odd, another route to customize the page is to head over to: Customize > Screens - and the screen we are going to customize is CRM_EditCustomer. Highlight the screen name, make a copy and when you highlight the copied screen you will now have an option to Edit.
If you follow these steps and still aren't able to customize the screen, please give us a call on the Method support line at 1.888.925.6238 x 2 and we'll be happy to take a look with you.
- Mortaza