There is also a Square Pie outlet at Spitalfields.
Doggetts pub on the south side of Blackfriars Bridge has a big pie menu (I could be wrong but I think they "outsource" their pie menu to the same people as The Blackfriar....menu looks the same...but they are good).
The recently opened Mother Mash on Leadenhall street does realyl nice pies...but from memory their checken pie is a chicken, ham & Leek.
I love chicken and mushroom pie too and can now vouch for the chicken, leek and mushroom pie on the BBC website. Made it tonight and it was yummay. I did cut back on the 5 fluid ounces of olive oil tho! Figured that had to be a typo. Also cheated and used a store bought shortcrust.
As Clarice mentioned earlier, both doggetts , and the blackfriar source thier pies from Brakes frozen foods.
this is the same as The Mudlark and hornimans ( also in SE1)
they are all part of mitchells and butlers "nicholsons" brand