Ok, so I've scoured the internet for frame sizing info and found an abundance of stuff for road riders but mountain bike seems a bit hit and miss. I have been riding mtb for probably 8/9 years and have always felt my bikes are too short. my current ride has an effective top tube of 24" and I use a 70mm stem because I like the direct steering feeling. It seems to me that frame sizes increase in seat tube size but the effective top tube seems to hover around the magic 24" measurement. Has anyone got any thoughts about optimum cockpit length for all mountain riding. I'm 6' 5" with a positive ape index, my finger tip to finger tip is 6'8".

LOL at the user name! I'm 6'2" and I normally ride bikes with a 24" toptube so I can understand why you think it may be a little short, I like Gary Fisher bikes as they have much longer top tubes than normal with their genesis geometry.