If your Desire is synced to Facebook contacts and some of your numbers are going crazy, I found this simple solution and it seems to work.

I have a contact with a mobile number of 070000000 (You get the idea)
Now if that person has entered there number on facebook, they add the country code so the number becomes 4470000000.

When the accounts are linked together, the facebook number takes priority.

Try to ring this and the number fails as it is not a valid number.

After playing around, i have found a way to fix it (yay i here you all say)

Click on Phone

Find a contact with a incorrect number (as mentioned starts 44)
Long press on the contact
Then click View contact
Scroll down and press edit
Change the mobile (This should have the proper contact number in it) to another option (Fax for example)
Click Save
Long press on the changed option above(Fax) and select "Set as primary number"
Select edit again
Change the Fax option back to mobile and click save.
This will make the proper number the default again rather than the facebook number.