(5 hours ago)F114TML wrote 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?

You could even drill it down further....

60x - Tyne Valley (South of Tyne)
61x - High Spen/Chopwell
62x - Local Hospital Services (Prudhoe/Norman's Riding)
63x - Blaydon Bank/Blaydon Burn/Winlaton
64x - Shibdon Bank/Winlaton/Swalwell
65x - Whickham/Lobley Hill 

66x/67x - Don't really recall any services in this range.

68x - Tyne Valley (North of Tyne)