Thank you Leonf,
Yes - the tensions disks are clean as a whistle, as is the whole assembly (I took it off and cleaned every piece - I took the bobbin shuttle apart and gave it a thorough clean too). The bobbin tension on a little Postage Scale thingy is reading about 1.75 ounces when the thread starts to move and even still I have to get the regulating nut on the tension post almost wound off the post to get a balanced stitch.
With the info I've been given here, and some more from "Andy Tube" on youtube on his vid about "zeroing out tension" I'm going to have a big play with it and when I work out what works nicely with this one I'll post my findings in the hope they'll help anyone else with the same problem. (Unless, of course, one of you clever folk come up with the Golden Solution or the readings that are working well on your 128 for top and bottom tensions first!)