Extra train sources by FGW▸ is a 142, but this extra trains will be operated with its brothers and sisters out of Exeter and will displace something else northwards to Bristol ... most likely a 143. Not sure how much difference there is, except that the 143s are being refurbished and the 142s are probably not.
Not possible. All FGW's Class 143 units are based at St Phillips Marsh. See list below.
143603
143611
143612
143617
143618
143619
143620
143621
What Andrew actually said was that
the extra unit in service on the Severn Beach Line wont always be the Class 142 unit. Perfectly reasonable , given that most (if not all)
TOC▸ 's rotate the units used on a given service each day.