I recently bought a Blackberry 8700g on O2, I have now been told by a friend that I cant purchase a Blackberry Data Sim Card without buying a Handset with it,

I just want the line rental Data Sim Card so that I can use my Blackberry properly,

Can anyone confirm this is true or not?

Cheers

I'm also an O2 UK subscriber and I was told by O2 that it is not possible to by a sim-only Blackberry data plan.

You need to simply purchase a Blackberry Data Plan from O2. The BDP does not "attach" itself to a SIM card, like a phone number would.