Broadly I believe,
1-99 Newcastle & Gateshead
100-199 Wearside
200 - 299 East Durham
300 - 399 North Tyneside
400 - 499 Washington (I think)
500 - 599 South Tyneside
600 - 699 Hexham, etc.
700 - 799 West Durham/Derwentside
800 - 899 Not sure
900 - 999 special/workers

Anyone able to add/ammend?