I have recently converted a couple of Hornby Black 5 models to P4. I have used a set of Ultrascale wheels on 44805 re-using the Hornby crankpins which enables the direct use of the Hornby valve gear. I had to open out the 3mm ID bearings and enlarge the slots in the chassis but the conversion has been successful.
For the other converted Black 5 45263 I used the 3mm Gibson wheel set conversion which proved slightly easier.
Here is 44805 on a test run. 45263 is on the shed exit road along with other recent conversions Patriot '45519 ' Lady Godiva and Jubilee 45682 'Trafalgar'.
The latter two are Bachmann models with Trafalgar converted to a LFB engine using part of the firebox and boiler from an old a Mainline model. It still needs a coat of BR green. Typically a week after finishing this conversion Bachmann announced a new LFB Jubilee!!!