Thanks DD. The rattle only occurs with the baffle (and therefore the nut and bolt) in place. Can't hear it with the baffle out, at least not over the noise of the exhaust anyway.
Rode it to work this morning with the baffle in and the nut and bolt done up as tight as a sardine's you know what. Wanted to make sure that it wasn't the nut and bolt rattling around. Rattled when I set off, not rattling when I got to work. Not coughing or spluttering either.
Your suggestion of turbulence noise is, I suspect, the cause of the apparent rattle but I'm still a little concerned with the coughing and spluttering. I know after market exhausts can cause pops to occur on overrun because of the PAIR system but this is spluttering when just on idle.
I'll see what it's like on the way home tonight. I'm afraid I'm a natural born worrier. Software I can deal with, mechanical stuff leaves me scratching my head looking dumb.
Thanks for the advice.