First Group own a relatively small fleet of Class 43 units.
Taken from therailwaycentre official publication about the MTU engine refurbs from a few years ago i count 13 Class 43 out of a fleet of 117 Class 43s for FGW:
43092 43093 43094 43097 43098 43122 43153 43154 43155 43156 43158 43194 43198 (yes 13, please feel to double check that as 13 does sound odd!
http://www.therailwaycentre.com/New TRC Main Pages/MTU Class 43s.html)
So i dont really think the Government is worrying here.
The new Greater Western franchise will be given to the company who gives the best cost effective proposal with perhaps a bold improvement programme, as envisaged from these newly proposed 15 year franchises.
I would imagine that the winning bidder shall be the company that proposes to replace all HSt stock, ie. SET Bi-Mode units for Penzance/Plymouth.
If your above worries about First Group holding this "trump" card of a silly number of 13 Class 43s out of 117 (11% of stock) to hold the Government to ransom to give First the Greater Western 15 year franchised. Do not be worried now. Plus i do think, currently First Group lease the HSTs to FGW. Well, First Group would just lease the HSTs to the new winner!! The idea they would take them somewhere else is silly. The Government say what stock provisions the franchises have, there would be no where else for the HSTs to go without the Government approving it.
Sadly these HSTs, whether you think they are great or old. The have little value considering they shall be scrap metal from 2016 onwards.