So, this evening has been a case of six and two threes. Decided to ride home from work with the Fuel cap cracked open (Not ideal i know, but quiet roads and just over a mile), the bike started fine but again under heavy(ish) acceleration, the but really starts to splutter, it generally happens around the 8-9k mark and is quite forceful. I don think it's the brake discs as you can here the Engine noise change when it dose it, it's almost like the engine looses power for a split second.
Got home, got the tank off and had a good look in there with a torch as suggested, but it generally looked in good nick and no real crud collecting in the bottom.I Liberated some pipe cleaners from work, and gave the vent spout a good clean, there was a bit of crud in there but nothing major. Put everything back together, tired to crank it, no choke, fired first time. Happy days. Put everything back together, went out for another ride. Still the violent splutter when accelerating hard...When i got back striped the tank of again and took the petcock out to to check that the filters were not block, all looked ok.
I know this may sound daft but is there anyway to test if the fuel filter before the fuel pump is duff other than replacing it?
So it looks like the issue of not being able to start with out the fuel cap open has been fixed but the stuttering still remains.
I'm not really sure what to try next (or indeed what is going on here)...as always, all suggestions gratefully revived.