Simon T
Welcome!
As has been mentioned all radios will have to be upgraded (replaced) to 8.33 by the end of 2017. Several of the new 8.33 radios (eg Trig) have built in intercoms. Assuming you are on the G reg C of A you could well be better off changing the radio now and getting the intercom included.
Had you been on a Permit to fly you could have done all the work yourself and just got it inspected and signed off.
Rod1