Yes, I have that in a Hornby L1, its a good sound and I like the variable load on F5.
The trouble is, I don't want the two to obviously sound the same and the N2 / L1 sound project has some fairly distinctive whistle clips. There's obviously no real J50 to record from (yet - www.facebook.com/TheJ50Group and http://thej50group.wix.com/home ) but having done a bit of research I thought the J94 from Paul Chetter / Digitrains might be be the closest:
J50 boiler diameter 4'5"@ 170psi pressure,18.5 x 26 cyclinders and 22963 lb tractive effort
J94 boiler diameter 4'3" @ 170psi prssure, 18x26 cylinders and 23870 tractive effort
I'm not sure about the whistle, having trawled my dvd collection and the internet I am yet to find a clip that has a J50 whistle and short of a trip to the NRM to look at the drawings, I'll assume it to be fairly high pitched in line with other contemporary LNER locos.
Another option might be the Digitrains / Paul Chetter D16 sound project which has the new braking on F2 settings... A trip to Lincoln and a play might be in order I suspect...
I know locoman has done a recording of a J94 but I want to save that for the DJM model.
Edited to add J50 group links