As a lover of all things vintage BSA, I think the story is perfectly believable.

There are a few things that people need to remember

1) The Mil Patt is a legendary gun, and really does possess legendary power

2) the .177 pellet would have been needed, as it flies straighter, and is smaller to sneak in between the metal panels of the 109

3) It probably had a full length Titan Spring in it

4) No Chronographs in those days

5) With a Spit on his tail, the Jerry pilot would have been nervous and hot, so is almost certainly gonna have his side window open for more ventilation. That will be where the pellet got in to the cockpit.

6) The ME109 looks well within the published 60 yd range.

I rest my case !