The Stanier Pacifics were in general use on the WCML sleepers until c. 1962, with EE Type 4 diesel electrics (later Cl.40) gradually taking over.
This period was also a time of chronic failures with the Type 4s' original train-heating boilers, hence a few 8Ps being steamed on standby until their own end.
I think the Princesses finished altogether* in 1962 and the very last* Duchess run was only 2 years later.
- Except, of course, for the present preserved examples which do not feature on scheduled sleeping car services.
(Princess Elizabeth herself ran south through Winwick this morning, en route from her base at the ELR, Bury, to Toton to haul a Saturday rail charter. There is a picture of her on newsgroup 'alt.binaries.pictures.rail' showing her passing Winwick under the heading:
"UK:Steam - A Low Light Lizzie today :-("
Should still be there if you want to take a peek.)