Rourke don’t fillet braze – tig!
I don’t think their mtb frames look right imo.
You’re going to find it hard to get more than 100mm suspension corrected frame, especially in fillet braze. I don’t know exactly why that is.
I’ve an Enigma fillet brazed mtb, it’s made of Columbus tubing, but they will do 853 that’s what custom is all about. If you google my name I’m sure I’ve posted many pictures of it, or google the STW reviews – it’s the actual one that I’ve repainted myself.
There is a guy called Kevin Sayles who now works at Woodrup Cycles, he does some mind blowing custom frames, not sure how clued up he is on mtb geometry though. Unlike road bikes you really do need to know what you want in custom mtb frames.
Shand Cycles do some nice fillet brazed custom mtb frames, they’re Scottish – not sure if falls within your brief or not.
From my own personal experience – don’t go to Croydon for a custom mtb frame – that’s all I’m saying. 😉