I've given up with seat reservations on the Portsmouth-Cardiff route and usually just sit in coach C when I don't have an advance ticket (and when a coach C is provided!). Very few people actually observe the reservations and most reserved seats are not used by the people who reserved them. They also cause delays with boarding, when some people spend a lifetime reading the ticket on every seat before deciding where to park themselves.
Whilst I often have a reservation I don't choose to sit in it since when I travel late in the evenings there are often better seats than my reserved seat which is invariably always the wrong way round. I personally try to avoid Coach C, merely because the chances of getting one of those awful 'Richmond' seats is far higher, where as most of A&B have the old BR seats with about a foot of padding to relax in!