Actually, I've just noticed that, rather confusingly, there are TWO links which allow you to find parts.
The first, and the one I was referring to when I started this topic, is on the left 5 lines from the bottom of the each page and you enter the part number you are looking for and a search tells you if it the part is available, its price and somtimes a picture.
The second, is when you click on an item in the online shop catalogue and to the right of the picture of the item two lines below the "Add to cart" link is a link "Show spare parts". When you click on this, a window opens and shows each part with part number, price and sometimes a picture of the part.