So the front wheel is off. I also removed the axle to expose the hubs housing the bearings. It was mighty grotty and gritty, explaining why the wheel didn’t turn very freely.
Next to come off was the rear wheel. A bit trickier than the front wheel.
I am not sure if this is a ‘cassette’ or a ‘freewheel’. From my reading it seems that the two are different in the way that you would remove them and the tools you would need… none of which I have. I also realised I need a special tool to get the axle and bearings out to clean them, but I don’t know which kind I need. More googling required.