I remember once seeing a Della in P9 at the Cross. Probably Grand Central, but I'm not sure now.
I've seen a HT 180 at P9 quite a few times in the evening peak - from about 18:15 onwards (I usually park my bike up there...)
What determines when P1 at Peterborough is used? From the looks of it, it can only fit 4 cars, and presumably isn't accessible towards Whittlesey, so I assume it's only ever used for off-peak FCC trains. It used to always seem to be in use at weekends (really handy when coming back from up North!). Admittedly more and more trains are 8-car off peak now, but FCC only ever seem to use 3a (except evening peak, or if a wayward Lincs-bound 153 blocks EC).
Likewise the bay platform at Huntingdon. Do the couple of daily Huntingdon terminators use it?