I've been back to that Citybond website Maggy and there is a drop down on the right hand side to select the trip duration. If you use that to select a 4 month holiday, so 3 months in SA and up to one month on the cruise, the cost rises to either £768.34 or £1074.85 but again, with no previous medical conditions reported.
Here is another site that you could look at:
International Health and Medical Insurance - IMG Insurance
As you seem happy with your current insurance arrangements for the SA portion of your trip, you could always consider just insuring the cruise to keep the costs down.
As for your question about how others manage on cruises, to be honest, I haven't a clue but suspect that some do not bother. Before retiring I had three years experience helping people who were unfortunate enough to be hospitalised on holiday. I used to call it their version of a two-centre holiday; one week beach and one week hospital! You cannot believe how many either did not bother with insurance or lied about previous medical history. Very stupid really as it can turn out to be so expensive. I had a lovely old Welsh guy once that had spent his life working down the mines and paid for it with emphysema. His wife took out the insurance and neglected that little detail. He fell very ill in Cyprus and the cost, including medical repatriation was over £65,000 meaning they had to sell their home to pay the bill. Another young lad in Egypt broke his back diving into the shallow end of the pool, had no insurance and had to pay for his hospital treatment and an air ambulance back to the UK. I dread to think what that cost but he will still be paying that back for sure.
I know that you will want to do the right thing though but, with your pre-existing, I'm am glad I'm not paying the bill! Mine is bad enough thank you, with a clean bill of health.
