I assume that 142's will take over branchline duties currently covered by 150's/153s so they in turn can cover for the 158s leaving to operate on mainline duties.
It should be remembered that an
FGW▸ head office "insider" has denied this.
From Insider (link below.)
http://indefenceoffirstgreatwestern.blogspot.com/2007/08/reader-mail-vol-iii-wide-world-of.html"Lee, tis is not strictly supposed to be in the public domain at present but it seems that Modern Railways have gotten hold of a snippet of information.
Thankfully, they are wrong (and not for the first time). Although there will probably be up to a dozen class 142s making an appearance shortly, and although they will be covering for
DMUs▸ away for refurbishment, they will not be working any further west than Paignton, mostly for the reasons you mention."