[Arran]

Mike;
  Sometimes you can get the cardboard sleeves to come off with a little heat, they are often held on with a blob of tar in the top. About 5-10 minutes in a toaster oven at around 200 degrees Fahrenheit should do it, a heat gun might work too but the risk of scorching the cardboard is higher. If that doesn't work then you may have to soak the cardboard in lacquer thinner to dissolve whatever alternate cement they used that won't let go with heat, the cement they used to hold tube bases on the glass is like this.
Regards
Arran