I'm in gillingham Kent.It is a can of worms , sorry
Only one fixing bolt, means the thing can twist and catch on the piston, You might be able to put both fixings in and tighten ,and it could work perfectly.
Where are you , ? I have a spare stepper you could try, it might just fire up and run perfectly.
They are so touchy, one tiny thing wrong , and it is a bag of shit. Get it right and they are really a fine bit of kit.
As you can see, there are some good guys out there , and you can fix it , do not despair .
Do not go down roads that mislead you , follow , what went wrong , what you have done , the results and do one step at a time till you find the solution
Hang in there , you have a lot of support .