Guys. Quick update
I tightend the head set, it turned out to have a little bit of play in it. I also reduced the play in the front wheel bearings.
Add to this I swapped out the laid back easton EA90 seatpost for a two bolt vertical EA90 seat post. This puts me further forward on the bike and in the position I more normally use on my other bikes. Interestingly the comfortable saddle height went up 1cm.
Then it rained cats and dogs and blow a gale for the past few weeks so that I didn't come down the hill at anything over 40Kph.
Yesterday was dry ish and not quite so windy. Came down the hill at 55kph an no real wobble, OK a little bit which having my leg against the top tube and a lighter grip on the handlebars never built up into anything other than a hint it might happen and it could all have been in my mind anyway.
Couple of issues still remaining. The Mavic Carbones get blown round in the wind a far bit and I think this coupled with the loosish headset and wheel bearing might have been the main culprit.
Also the handlebars are carbon fibre and have a bit more flex in them than I am used to so I was gripping them to hard.
Remaining changes are to remount the break levers higher up on the bars. This will throw me into a more neutral position and more in control of the brakes both when I am on the hoods and on the drops.
I've learnt a hugh ammount over the last couple of months re bike positions and fit etc: so thanks for all the help.
Now as spring gets started I wonder if I can top 70kph down this hill without wobble.