many thanks to all those who replied. The general concensus is that just 600mm of topsoil cover might not be adequate but it also depends on the amount and type of contamination. Some with arboricultural experience also commented that for medium sized trees a good 2m of good quality soil is required. Of course everyone commented that the vapour pathway could be the predominant exposure route. I should have mentioned on my original memo that the TPH levels are up to 30000mg/kg (and at depth up to 2.5mbgl). They will put a HC/vapour resistant membrane for the buildings.
I think that a lot more information will be required on this subject before I accept the magic number (which the report states that is the standard BRE guidance).
Once again thank to everyone for their help (Albert thank you for the attached docs I am sure they will make an interesting reading).