Called National Rail, who reliably tell me I have to travel back to London, buy a Swindon return, then travel on my original ticket from London to Bangor.....
WHY!?!?!?!
Can I not do the Didcot / Swindon return, and then travel to Bangor through W'brom or B'ham and Crewe?
Help, please?
I'm sure one of the Fares Advisors will be able to confirm but that advice from National Rail is not correct.
Bangor has one routing point - Llandudno Junction.
Bognor Regis has 2 routing points - Havant and Ford.
The Anytime Single (used to determine valid routing points) is the same price (£187.50) from Bangor to Bognor Regis/Ford/Havant so either routing point is fine.
Havant to Llandudno Junction gives valid routes of:
PD+EB+BC+CH
PD+EB+BC
LP+EB+BC
LP+EB+BC+CH
LONDON
LP+EJ
PD+EJ
RB+BC
RB+BC+CH
RJ
XS+BC
XS+BC+CH
XS+BJ
For completeness Ford to Llandudno Junction gives the following routes:
SC+EB+BC+CH
SC+EB+BC
LB+EB+BC
LB+EB+BC+CH
LONDON
LB+EJ
SC+EJ
The only map we need here is map XS which shows us validity between Havant and Birmingham (BC allows you to continue from Birmingham via Shrewsbury, BJ via Crewe).
Map XS shows a valid route Havant - Guildford - Reading - Didcot - Banbury - Birmingham. (as in fact do maps RB and RJ).
I believe this means that your ticket is not valid to Didcot via Horsham, Eastleigh or Southampton. However it is definitely valid via Havant and Guildford and most of the day this looks to be the quickest route anyway.
Just put in via Petersfield to restrict itineraries to your valid route. The nationalrail.co.uk journey planner will give you a valid itinerary for this route on a single ticket which is definitive proof the route is valid if you're concerned.