I’ve just got back to the UK, the modem which is on 3 does pick up the Tre ITA signal, however I couldn’t log on even with the international barring removed.
After about eight calls to 3 back in the UK ,I finally got through to someone who actually knew what they were talking about, yes I was surprised by that!
I discovered from the phone call that all modems need the International baring removed (which I was aware of) AND also need the modem to be activated by 3 to be able to roam abroad.
They’ve informed me that this has now been done, I am going back over in six weeks, I'll let know how I get on!