thanks for the tips. well, back from the suspension shop and feeling like an idiot.... doh!
turns out i simply needed to bleed the air out of the forks. the pressure was making the forks top out more than normal and they said the design of the sachs forks seems to have slightly longer springs than probably needed which makes them top out a bit more than the average fork. they reckon with decent top out springs there should be no issues, but agreed that if the 2015 springs and earlier were low quality jobbies then it's not surprising if they are failing on some bikes. they are getting the later model springs and will fit them for me next time i'm up there.
moral of the story?
bleed the forks regularly.
if you get that clunking sound regularly with 2015 or earlier sachs forks, stop riding. your springs have probably gone and will mash your shims etc. get the 2016 springs.