On having a look through the relevant Yeadons (vol 27) it would seem that N7s started to be moved away from Stratford in 1948-49 to locations such as Colchester, Parkeston, Ipswich, Norwich, Hornsey and Neasden amongst others. Although this was only in small numbers of 4-6 to each location and they were frequently swapped back to Stratford and replaced by other N7s. By 1951 a few had spread further to Annesley, Colwick and Bradford.
Larger quantities of N7s began to be moved from Stratford permanently from 1956/57 and by 1959 the Stratford allocation was down to 61 from over 100 in 1950.
In LNER days the bulk the of changes to allocations seem to be between Stratford, Hatfield and Kings Cross, with a few going to Neasden for a short time in 1925/26 and Cambridge in 1933. During the war a few were allocated to Bradford.
This is of course a very broad summary have a look at Yeadons if you can plus this site
http://www.stciers.me.uk/home/sheds/index_sheds.html
is really useful for East Anglian allocations.
Cheers