On my old Front Derailleur it says 31.8 but I can only find the seat post I want in a 31.6 infact they all seem to be 31.6 Am I to asume that 31.6 is for a 31.8 tube?

The long answer: 31.8 is the outside diameter of the seat tube. The seatpost has a diameter (pretty much) identical to the inside diameter, which will obviously depend on the wall thickness. You need to look at the old seatpost to be sure. 31.6mm is massive, I'd be surprised if it was one of those. What frame is it?

The short answer: no.

Its a 20" frame Carrera Vulcan frame (Merida Mats sport, I THINK) the old seat post got stolen.

Front mechs only seems to come in 31.8, and 31.6 its 1/5 of a MM smaller thats about a hairs width. seems strange that loads of 31.8 Seat posts are made and no 31.8 seat post.

?

indeed

Seat post will certainly be a lot less than 31.8

You'll need to get it measured or ask the people you bought it from what size it should be

If it's an ally frame then a 31.8 seat tube tends to correspond to something like a 27.0 or 27.2 seatpost.

As Alan says, you can't guess it. Measure it.

Or borrow a mates post and go from there.

go into your local bikeshop or halfords and try some of their spare ones. They might be willing to help you out.

Phoned Merida it was 27.2mm

Incidentally, Trek MTB's and Principia road bikes amongst others have 31.6mm seatposts. Front mechs are all either 28.6,31.8 or 34.9mm.