I am a British citizen owning a house in South Africa, I intend to travel to South Africa for 8 weeks this month, return to the UK for 10 days approx at Christmas time and then return to South Africa for around 3 months, is there a defined time that I must be out of South Africa before returning?
You can stay 90 days in one go. The issue in your case is that you will return to SA before that visa expires. Technically, your second entry will be still on the first visa. There is a chance that they will not even give you a second 90 days visa. This thread here talks about this issue.
Further, same same applies for a single-entry residence permit:
I'll be back soon - will I need a re-entry visa? If you are in South Africa on a valid permanent residence permit, and you leave the country temporarily, you - and your dependents, if any - will NOT need a re-entry visa, so long as your permit remains valid. This holds even though your permit may have "single entry" endorsed on it.
You best bet will likely be to exit the country, come back from the UK within the 90 days for your first visa and state the time of stay at immigration. If they just give you a second visa, you are good. If not, apply for an extension of the visa at the Department of home affairs.