This may be of interest
https://www.gumtree.com/search?search_category=all&q=rossin%20racing%20bike&search_term_populated_by=input&keyword_correction=auto&search_location=Norwich
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!