Tony Polson wrote:

Bearing in mind that you are in the UK, a reasonable cost to repair
this lens would be about GBP 30 to 40. However, that is probably a
lot more than the lens cost you. Doing it youself might be an option,
but inexperienced hands can do a lot of damage to a perfectly good
lens.

Personally, I would return the lens to the seller and demand a refund,
then find another example that doesn't need work.

I would strongly recommend that you should look for an SMC Pentax-A
50mm f/1.7, which is one of the best 50mm lenses ever made for an SLR
of any brand.

That is indeed the one. I got it for less than £40, including postage.
It's a really excellent lens, and takes great pictures.

Daniele