The exhaust does seem to be doing a little splutter every now and again, it does sound a little burbly now. I don't think a gasket would make a knocking sound like this though.
Unfortunately I've been having to drive it this week as I don't live near my work. As the noise isn't noticeable during driving and doesn't seem to make any difference to performance I've just been keeping the revs down low and taking it steady. It's also helped me pinpoint when it's making the noise.
The guy looking at it on Monday is someone who's worked on all my cars, he also picks them up which is good as I'm always too busy at work to get to any garages during working hours. If his verdict isn't good, then it's staying with him and I'll be finding alternative transport into work!
wish i;d taken a sound clip of my bike (SV650S) before the valve shot straight into the front cylinder piston... then afterwards it sounds like you could hear the valves through the exhaust pip... (splutter bang bang)