Could also be that one, or both, cranks are not tight enough on the spindle. I'd check that out too. I'm sure there'll be a member with more knowledge than I, who'll be able to tell you what torsion should be used.
Every excuse I'm sorry to say. I'm just buying a car (!!!) at the mo. May be able to consider this in a week or so. I'll pm you .
I don't often venture as far afield as Suffolk. Let's see how the derby game goes!
Try a "cone point" grub screw. Lots on Ebay. You shouldn't have to drill the seat post but ideally drill and tap the frame or collar. Bit of red loctite, job done.