Mark.
If your father was firing 5.5 inch guns then he would have been in a Medium Regiment Royal Artillery. These are distinct from Field Regiments and Heavy Regiments. They are also distinct from Survey Regiments, Light & Heavy Anti-Aircraft Regiments.
They are normally given a number – 56 Med Regt RA.
Knowing that he was in a Medium Regiment seriously narrows down the search.
Regards
Frank