I may beable to get my hands on a valve body from a EF V8 (BTRLE97). My current trans is undergoing some problems when it gets warm with flairing etc and i think the S5 is dead or other solonoids. Id like to replace the lot in one hit.

Are the V8 and I6 Valve bodys interchangeable? Cheers

Check the part numbers stamped on the valvebody lower. Will give you your answer. Otherwise why not canablise the V8 one for parts.

Most shifting problems are caused by not enough fluid or fluid pressure or faulty S5 espeically if temperature related. Also old black fluid (can we see a pattern here?)
If all that is good then it is a mechanical failure, either clutches, bands or pistons. - Or you are trying to put 600Nm through it!