Buzzard has correctly identified it as a Long Tom (155mm Creusot, french cannon).

It is in South Africa but not at Mafeking, Spion Kop or in the province of Natal - all are great gueses (except Ed's).

The location of this cannon will be revealed later if no one gets it. I will say it is in a very picturesque area of South Africa - an area full of history where I used to work and hunt.