Davef68, I got a copy of my father's merchant marine records from the Marine and Coastguard Agency at Cardiff and they helpfully translated some for me too. I don't think they even asked for a fee. I am not sure but the records may have been transferred to Halifax, Nova Scotia.
PS, I see WD's link is to the MCA. To expand on what was provided, I got a copy of the signing on register for the British sailors and their rate of pay for each ship he served on. Using other sources checking the war service of each ship he had been on showed that he did many short hops, Liverpool to Cardiff for instance. There were actually very few when the signing on period coincided with a particular convoy. This is at odds with the number of theatre awards that he had - Burma, Pacific, Atlantic, France and Germany, and Italy. I could trace the probable qualification passages for all apart from the Italy Star.
As far as Arctic Convoys there are lists of ships that made that perilous journey. Perhaps more US ships than British and a fair number of foreign too. This list will help you focus on your grandfather's. Do you know which company he was employed by?