I found out just now that seafoam is not available in the UK! Yay. I bought some redex cleaner, Im not sure if thats the same principle as I just lob it into the fuel tank, but its supposed to clean it up.

 

Just for info, it turned out that it was just the oil sensor (woop!), they changed oil pressure switch and checked the pipes for leaks, and there were none. The rattling was the catalyst which is a bit loose so im not getting much bite in the accel as normal. There a a few other problems but nothing integral. The whole lot cost me £300 which was a lot, lot less than what I was expecting.