I'm pretty sure that any correspondance in the case of PFxxxx is not subject to FoI due to data protection issues. You may possibly have joy instead with a Subject Access Request under the Data Protection Act if the case involved you though.
As for Transport Focus generally, my opinion of them is very bad. I'm been very unimpressed with the correspondance I have had with them in the past, when they were called Passenger Focus. Various FoI requests to them on whatdotheyknow.com also show that they have a severe lack of understanding or inclination of the NRCoC or Routeing Guide, and are more interested in fobbing you off.
I've also seem some correspondance for other peoples' cases. In one case, after months of correspondance during which they totally failed to grasp the issue, they eventually found in favour of the passenger, but then told the wrong TOC to issue compensation, despite this TOC not being involved in the dispute at all. Another case made it all the way up to their then top dog Anthony Smith, who then wrote a lengthy and condescending letter to the passenger, rebutting almost all of the passenger's points. Sadly, most of the content of Anthony Smith's letter was wrong, and showed a total lack of understanding of the Routeing Guide, as the passenger was actually in the right.
Also, FoI and Data Protection Act SARs that I've seen show that they have a very cosy relationship with the TOCs and with ATOC. On numerous occasions, when a complaint about routeing has been made, instead of investigating things themselves, they have been shown to have just asked ATOC's view on what the validity is and then just parrotted this back to the passenger. They appear to belive anything ATOC says about routeing, and ATOC can construct some very misleading or downright false replies when it suits them.
Transport Focus don't care about the technicalities of whether you were in the right or not. They are more interested in getting passengers off the hook when they forget to buy a ticket or some other story that will put them in good light with the media. Try and get your escalated to David Sidebottom as he seems to be the most clued up employee they have who is the only one who is any good - however as he's now had a promotion in the new reorganisation I suspect he may not deal with individual cases any more.