(5 hours ago)F114TML wrote Broadly I believe,
1-99 Newcastle & Gateshead
100-199 Wearside
200 - 299 East Durham
300 - 399 North Tyneside / Northumberland Blyth + Seaton Valley
400 - 499 Rest of Northumberland
500 - 599 South Tyneside
600 - 699 West Gateshead / Northumberland Tynedale 
700 - 799 West Durham/Derwentside
800 - 899 Independent
900 - 999 special/workers

Anyone able to add/ammend?

Think that's the way it was wasn't it? Obviously Northern didn't run North of Blyth so didn't have anything upthere but it's generally how things were as far as I can remember between Northern / United, bar a few oddities.