I've found conflicting information on this.
No visa is required for Italian passports. For South African ones a visa is needed but is obtainable on arrival.
So far so good.
But one source says that passports must be valid for the length of your stay, and another says 3 months. Yet another 6 months. If the latter is right you are stuck anyway.
Getting back into Britain is no problem - your Italian passport just needs to be valid on the day you arrive.
Try:
Embassy of the Republic of Tunisia
29 Prince's Gate
London
SW7 1QG
Tel: (020) 7 584 8117
I haven't been able to locate a proper website for them. The one that comes up on google is a commercial site of dubious origins.