The HST▸ diagrams in my opinion are quite frankly hopeless and don't serve any of the real demand in the South West!
That is because the HST diagrams are not intended to meet the peak demand in the SW, they are timed (as the timetable shows) for the peak demand in the section through Bristol/Birmingham/Derby/Sheffield/Leeds, southbound in am, northbound in pm.
The requirement was described in the franchise briefing, and again in the franchise award announcement, as follows:
Bidders were asked to submit two proposals, one of which required delivery of a minimum of approximately 30% increase in capacity measured in terms of number of seats provided on the central, most congested section of the franchise. Arriva will deliver 35% exta capacity - nearly 3000 seats on key routes - by June 2009 with most of the extra seats being provided by December 2008.
Proposal A was basically 'do nothing' - Proposal B was as above.
Paul