Thanks for all the feedback in my previous post. I managed to sort the richness issue out by replacing the 280 main jet with a 150 main jet. The spark plugs are looking a nice brown colour now with no carb deposits. In fact, the motorcycle was running so well that I decided to go for a longer ride around North Wales.
The bike was running great for the first 45 mins but then the engine started to die whenever I gave it more throttle. Instead of increasing in revs and sounding amazing, it sounded as if it is was muffled and starved of air. Very bizarre. Both fuel taps were open but this didn't resolve the problem.
I also noticed a bit of oil to the posterior side of the cylinder base. There was no signs of oil dripping from the rocker boxes or cylinder head. The only explanation I can think of for the oil is that it has managed to get pass the threads of the cylinder base studs?
The oil tank was also low on oil but was full when I left the house without any major oil leaks being evident. Perhaps the pump is drawing too much oil from the tank and not returning oil at the same rate? So the oil is accumulating and wet-sumping the engine? I'm scratching my head on this one guys, does these symptoms sound familiar to anyone?