I strongly recommend you to take this to a reputable repair shop. Often used music stores have a guy they work with…
…If you insist on repairing this yourself, first contact Marshall and ask if they can supply you with the correct part. Send them the model number and tell them which part it is. This is you best and easiest option for DIY.
If they cant or wont help you in that way, ask if they will supply you with either a schematic w/ parts list, or can give you the correct part number for the volume control pot.
If that fails, its the hard road… you will need to remove the PCB, then unsolder the micro-pot from the PCB and inspect it for part numbers. Use a magnifying glass and bright light if nothing is obvious.
If you find no numbers you will need to do a manual part search. First test the part and determine the maximum resistance. Then using a visual match figure out what ‘package’ the micro-pot is.
locate a replacement, and cross-reference if necessary.