That's brilliant! Thanks so much again Flood. Looks like the easiest thing to do would be to use this coach as the basis for the TRSB - it's only the counter/kitchen area that's wrong. I could cut this area out and replace it with the same from a TRFB and that looks like it'll get me a true TRSB.
Looks like I'll have to find another Hornby 4 window buffet for my RFM then - or just wait until Hornby release one in Swallow which, if Hornby have any sense, can't be too long now given that Bachmann are doing a Class 90.
Just out of interest, why were the rakes organised such that the underframes all faced the same way? Was there any reason or did the powers that be have OCD? I've just noticed as well that when I try and do as you suggest in my rake and have all the underframes facing the same way, the seating area on the "TRSB" is facing standard class... one of the chassis must be the wrong way around! (Edit: from referring to the photos you've kindly linked above, it looks like the chassis on the "TRSB" is the wrong way around. Given how much of a pig Lima Mk3s are to take apart I'm not looking forward to sorting that out!)