Nokia Nseries :: Contacts Not Recognised In Messaging?

Over the last few days I've received text messages from some friends who have new numbers. I saved the numbers to their existing contacts, overwriting the old numbers.

Despite having restarted the phone several times and removing the sim card, whenever I look at my texts, the messages from these numbers only show the number they were sent from, rather than the contact's name.

I have had a look online and it seems that this problem is usually caused by duplicate contacts. I have used the phone's search function to make sure that each number only appears once, and there are also no duplicate contact names.

Some other advice online suggests that the area code could be problematic. I have tried changing the beginning of each number to 0 rather than +44 (and back again) - no difference, I am now at a loss and unable to tell who is texting me without using the search function, which is pretty ridiculous.