Previous owner of the house Mr MacPherson (who bizarrely was at my wedding - small world!) have now been in contact to say:
"I think that spear was an African one that my Dad had a bunch of from somewhere or other, he did have relatives that were in South Africa at the time of the Boer war so it might have been from them. I used to chuck them about all over the place when I got the chance so there is a strong likelihood that the one you found was one of those! Perhaps you had better tell Gordon so he doesn't get all excited about it..."