A screw was neatly embedded in the rear wheel

No problem I think, whip out an inspection light and set to said puncture with the on board repair kit (I'm a veteran of 2 successful and 2 unsuccessful repairs...) thinking I'll have this sorted in 20mins and ride down to FWR in the week to get a new rear - which I've been promising myself for a while.
Not to be. The rubber bungs although fresh out the wrapper decide they don't want to go into the hole (despite religious instruction following) and instead neatly shear in two every time I try to drive them home with the metal whotsit tool...
So bike will stay with flat rear until I can get round to removing wheel and driving down to FWR....
Pants.
Anyone had a similar experience with those rubber bung thingies. The ones that came with the bike worked fine, purchased the above kit from Hein Gericke, all the bits are identical - could the rubber have perished (the seal to the packet was good...)
Ade.
