I have fitted a set of new clutch plates but still get slip in the higher gears, 4th, 5th ,6th.Do not feel any slip in lower gears. There is free play at the lever & the action is light & smooth.Any thoughts? Also my rear brake plate broke at the swing arm location pin, is this a weak point or I am unlucky? My wonderful machinist friend made me a new one.

Try a very light gear oil or ATF re the slipping.The oil will disperse from the plates quicker. If this fails , replace the springs as they are probably old and saggy after all those years

Ok will give that a go. Then try some new springs if needed. Thanks

the springs are only a couple of quid whip em in job done.