Transmission1

The only frequency i have is 205.15 NFM. This is for the RETB system on the East Suffolk Line from Lowestoft to Ipswich.

I do want the frequency for the mainline trains from Norwich to London

I think you'll find they've gone to GSM-R (876–880 mobile and 921–925 base). Digital. No chance.
http://en.wikipedia.org/wiki/GSM-R

train stations are in the 453 area colchester north station is 453.550 with ctcss of 118.8
you can hear rail cabs on 454.925 area