As suggested by Phil i have injected silicon grease into the Centa unit, easy to get all the way in using a large syringe with a small tube.
Been out for an essential journey and the result is a noticeably smoother ride with less vibration felt through the seat. Surprised that a bit of grease could have that effect, wonder how often it will need doing though.
Still got the annoying rattle and knocking sound at around 3.5k rpm though that i would like to get to the bottom of but try to ignore!