i would think it advisable to have membrane and the usual air gap
incase you get a leak as theres no g'tee that the cladding will remain water tight
whats the structure behind the  cladding!!!

ok i ment the internal structure between you and the cladding!!!

Vapour barriers sit on the inside of the insulation and prevent warm moist air from inside condensing on the cold inside face of the outside covering. What you are talking about is called a moisture barrier and works in a similar way to roofing felt.

If I build a timber frame wall starting from the inside,

plasterboard

vapour barrier, (polythene sheet) or can be incorporated in the plasterboard if you use the foil backed type

stud wall with insulation

11mm OSB board (This one is optional but gives a much more substantial wall)

moisture barrier I  find it is best to fit a breathable membrane such as Tyvek or Rubberoid, expensive though. If you fit the additional sheet of OSB the membrane needs to be firmly fixed with staples or the wind gets underneath it and it makes a hell of a rattle.

Batten using tiling battens, as this will provide an air space

external finish

Even though the plastic cladding is waterproof, I would still fit a moisture barrier


Hope this helps

Thanks for the advice woodsmith. I have now got some Tyvek. I am going to use foil backed plasterboard also.

Not going for the 11mm board, no room!

When using the Tyvek, how is it best to join the separate pieces of Tyvek, the stud wall is going up in three 2.5m sections.

Any other tips?

Thanks for all the help
Simon

I will re-look to see if I can squeeze in the OSB board.

I presume I can use the Tyvek under my tiles?

Thanks again!