There are 3 people off the top of my head who can cover you coaches wise. Wagons I can't advise on.
Bowaters Models has a popular range of 10 (soon to be 15) Modern FR coaches which would be perfect for the Roundhouse loco (DLG being the youngest of the fleet (for now)). They also have announced a range of 5 heritage coaches due next year. Excellent coaches and very good value for money (but I'm biased so......)
http://www.bowatersmodels.co.uk/ffestiniog-railway-fr/
IP Engineering has mostly the smaller stock covered with a range of Bug Boxes and other smaller coaches. Again, excellent models and well worth the money.
http://www.ipengineering.co.uk/page6.html
Lineside Hut does a range of more vintage coaches which again, I believe are due to arrive soon. Only company here I haven't either bought from or own it myself so can't comment but I've only ever heard good things about them!
http://www.thelinesidehut.co.uk/ffestiniog-coaches.html
Images from respective company websites.
Hope that helps!
Jon