Drained tank. Crappy fuel and did not need a fuel anylizer to figure that out. What a pig of a job. Fitted fresh fuel. Turned out that the intermitent rough running was EGR valve broken. Just goes to show how you can get caught by a coincidental problem and how one thing can throw you off the scent.
PS. Has anybody noticed that manufacturers seem to be putting things under the bonnet with sharper and sharper edges on them. Still, every time I cut myself I stick an extra fiver on the bill