Expansion tank bladder pressure?

I have a Worcester 350 combi which I have to pressurize it to about half a bar before it fires up, been told that the expansion tank bladder is gone(needs replacing) or deflated and needs recharging(when I press in the tyre type valve on the expansion tank,water comes out and also water dripping from the downstream of the 3 bar relief valve to the drain outside the house).
If the bladder is deflated,what do I use to inflate it air or Nitrogen(in my job, we use Nitrogen gas to inflate bladders) and to what pressure?
Is changing the bladder a major job?