Re the initial post, the overwhelming majority of Sunday services on the Cotswold Line, as it is still known locally, are now operated by nine-car IETs, so there are plenty of seats available in first class. As noted above, since you are boarding at Great Malvern you will be spoilt for choice, whether it's a nine-car or five-car set.
Different Cotswold Line from that refered to by the OP!
At one time the term "Cotswold Line" only refered to the Hereford - Malvern - Worcester - Oxford line. Hence the Cotswold Line Promotion Group (CLPG) is the rail users' group for that line.
Later, at some stage GWR (the modern reincarnation) started to refer to that line as the "North Cotswold Line", and the Gloucester - Kemble - Swindon line as the "South Cotswold Line", hence the confusion.
To be fair, they both serve the geographical area known as the Cotswolds, just different places within it.
Strictly GWR serves another line in the Cotswolds National Landscape (formerly AONB, area of natural beauty), as the line from Bradford on Avon (boundary is at station car park) through Freshford to near Bathampton is also within the NL. The NL boundary is right next to the junction at Bathampton.
So what they erroneously call South Cotswold line is closer to a mid Cotswold line, as they serve a line in South Cotswolds too.
Linking the Cotswold National Landscape boundary map
The term Cotswold Line - ie Oxford-Worcester - was first used for marketing purposes by local British Rail management at some point in the mid-1970s and was adopted by the campaigners who created the Cotswold Line Promotion Group in 1978. Its newsletter was rebranded as the Cotswold & Malvern Line News in the early 1980s, reflecting the services that extended west of Worcester and station namebaords and marketing material with a Cotswold & Malvern Line branding appeared under FGW in the 2000s.
The terms North Cotswold Line and South Cotswold Line were coined by Network Rail, not FGW/GWR. to differentiate between the two routes in the late 2000s and certainly by the time of the start of redoubling work between Oxford and Worcester in 2009, as there are press releases from that year referring to it as the North Cotswold Line. I've no idea what was wrong with the previously commonly used Golden Valley Line name for the Stroud and Kemble route.
While geologically speaking the area around Bath and Bradford-on-Avon is in the belt of oolitic limestone - aka Cotswold stone - that stretches from north-west Wiltshire across Gloucestershire and part of Oxfordshire - hence the National Landscape area designation - how many people who live there or visit the area actually think of it as the Cotswolds?
