So your train operators are Abellio Greater Anglia, East Midlands and East Coast. A single phone call to Abellio Greater Anglia's assisted travel team should be all that you need: 0800 028 2878 (free on a BT landline), 7 days per week, 0800-2200. Alternatively email
[email protected]
You shouldn't need to contact anyone else but, should you need it, the contact number for East Coast (who run the stations at Peterborough and Berwick) is 08457 225 225
Diss station (which is only 20 miles away from me) often only has one or two staff on duty, so arrive early to give them time to help you. (Although there are steps up to the Norwich-bound platform, you'll need to arrive at the main entrance on the London-bound side to let the staff know you've arrived and to get help taking your bags over the bridge).
The staff at Diss will phone through to Norwich to tell them which coach you're in, so you should be met when you arrive at Norwich, where you'll be given assistance to the East Midlands train (which quite often runs from the opposite side of the same platform that the trains from Diss arrive at anyway).
The Norwich team will then phone through to Peterborough, to arrange the next stage of your assistance. (You might find again that your 3rd train leaves from either the same platform, or directly opposite, the platform you arrived on. That's quite common when changing trains at Peterborough).
It's many years since I last visited Berwick station but I'm confident that, having been notified by the staff at Peterborough, there will be someone waiting by your coach to assist you when you arrive there.