UKH

I’m looking to apply for a Polish passport by way of descent. My Polish grandfather (sadly deceased >30yrs ago) came to the UK after WW2 from a part of Poland that is now Belarus. My grandmother is still around and has all his naturalisation and other papers. 
 

There are companies advertising services online that claim to take care of all the appropriate checks and paperwork but there are seemingly hundreds of them and they’re all overly SEO’d and pay per click advertised websites which feels a bit spammy  

Has anyone used one of these services and could recommend one? Do I even need one or is it a relatively trivial and idiot proof process to do myself?

No idea, but I looked into it for Ireland (obviously the benefit of no language barrier) and couldn't see any reason not to do it myself. In the end I didn't apply as there is no real benefit to me having dual nationality.

Two stage process, first apply for dual nationally. Then if approved apply for a passport as normal.

Unless it's especially complicated I feel that the sites you refer to are just cashing in on people's fear of burocracy. Although obviously for yourself if the application has to be in Polish, this is an additional complication that may justify paying for help.

One of my mother’s friends has gotten a German passport via her (deceased) mother being born in Germany. She applied via the German consulate and found the process straightforwards other than being process intense. It wasn’t particular quick but once the application was approved the passport was issued pretty quickly. I suspect it very much depends on the country and how keen they are to issue passports to UK citizens. 
Re an online service….I’d not be keen to give that level of information to a third party to do anything on my behalf. 

I have an Irish passport as my mother was born in NI. Just filled in the  forms and followed the instructions. You don't need someone to do it for you. Translation maybe, though?