"now to find who the dad was as it’s not my grandad…"
Have a look at who your shared matches are.
Go to your DNA Matches page and then click on this person.
This will take you to the page showing your connection to him. There are three tabs; 'Trees', 'Origins' and 'Shared Matches'.
If you click on the 'Shared Matches' tab then that will show all the other people who have done Ancestry DNA tests that share DNA with you and also at least 20 cM with him.
He can also do the same which may also throw up some additional people.
So, these are people who are related to both of you.
You will then need to find out where these people sit on your family tree.
If there are any joint matches that you cannot locate on your known family tree then they will be linked to the unknown biological grandfather.
You can then use these people as a starting point to identify who the unknown biological grandfather might be.
[EDIT]
Sorry, just to add to this. I've been in a similar situation.
I worked with a DNA match whose mother had been adopted at an early age. It turned out that the father given on the birth certificate for her mother could not have been the person named (he had died five years earlier).
After ruling out other possibilities we were left with it being her unknown biological grandfather who was the link between us.
I did a bit more digging and found a likely candidate who was in the right place at the right time, my great grandfather’s cousin.
It can be a lot of work to find out the relationship but it is certainly possible to do.