SP - yes, they're the ones. The reason that they're easy to remove is because you can buy 19mm ones from Maplin at 99p for a pack of ten.
However - the frame hole is actually 22mm so you need to do some digging around and find the right size plugs but unfortunately you have to buy them in packs of 200 direct from the manufacturer!
This is why you don't want to buy the ones linked above off ebay as they're the 19mm ones which don't fit too well and fall out.
Drop me a U2U if you want a pack of the correct size (3x22mm + 2x30mm) as I do good discounts
