Hi all thanks for replies, had some time spare today so pulled valve cover off and valve springs and checked height difference of valve guides,exhaust guide was about 1/4 higher in the head than inlet,so put a long reach 7mm socket over it and drifted in back down to the same height ,peened the metal around it in and put it all back together ,set valve clearances and fitted valve cover,fuelled it up and just run it for about 1 1/2 hrs cutting grass and it didn't miss a beat ( 35 min was usual time before it played up ) ,so that has cured that ,dont know for how long but will keep my eyes open for 2nd hand head but not very common find here in the UK,again thanks for your help guys much appreciated.