Thanks a lot for the input! Finally a decent answer hehe

I'll do what you said as soon as I get home today; the chain tension issue you raised was probably caused by me pedalling gently (with my hand of course) while the bike was upside down. I was doing this just to remove all the grime and dirt that was collected thanks to the lubricant I was using (motor oil).

Again, thanks for your advice and I'll keep the gear changing in mind for the future.

P.S. which component needs to be bent? Is it the metal body of the red pulley?