It sounds like it should be an Albion HR3 or 3A gearbox which was quite common on most of these vehicles.
First thing that I would look at in both cases is the clutch plates. The corks may be sticking to the steel plates with old oil or grease- or rust if it hasn’t been used for a while- and it’s the easiest part to get at. Just undo the hexagon headed shoulder bolts with springs underneath and it will come apart, but take care to catch the operating rod in the centre when you pull the outer cover off.
Once you’ve got the outer cover off you will see a series of plates, every other one will have corks in. One set of plates will have “ears” or tags on the outside which engage in slots in the outer cage; make sure that there are no burrs on the edges of the tags (file them off if there are) and make sure that the edges of the slots in the cage are smooth and not a series of notches where the tags have worn grooves. If you have notches file them smooth with a file so that the tags will slide up and down the slots without snagging.
Before re-assembly make sure that the clutch operating rod is working smoothly backwards and forwards. It is normally in two pieces- a plain rod where it goes into the gearbox on the clutch lever side and a rod with a foot on it that pushes on the inside of the clutch cover, and these two are normally separated by a ball bearing in the middle.
If cleaning the clutch up doesn’t do the job it may be that the input shaft and output shaft have become stuck together. The one shaft runs inside the other and if you didn’t want to dismantle the gearbox you can sometimes free them off by running the engine and selecting a low gear- 3rd gear is normally a direct drive with no difference in the two shaft speeds, but 2nd or 1st gear should show a result but make sure that you’ve drained all of the old oil out first and refilled with fresh oil so you don’t do any internal damage.
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