Hi Martina,
I’m firmly of the view that this sort of problem is the ‘elephant in the room’ with regard to SCTs. It’s been a constant issue for the 10 years I’ve owned my CPC1100.
Small specks on the corrector won’t be a big problem and, unless it’s obviously affecting your images I would leave well alone.
Steve Collingwood of SCTelecopes (and Pulsar Observatories) did a good job cleaning and collimating my telescope about 5 years ago. I’m not sure he still offers this service but try
http://sctelescopes.com/sc-telescopes-broadhurst-clarkson-and-fuller-fullerscopes/
With the recent demise of David Hinds I think Tring Astro might be able to advise.
I would appreciate knowing how you get on. Good luck!!!!
Trebor