Hi there and thank you for your question,
I will try to assist you with your legal question but please feel free to ask as many follow up questions as you like until you are 100% satisfied.
If the house is 40 years old, and you didn't do renovations in the past 15 years, then I really really doubt that the cracking is due to settlement and expension of the subsoil. You should most certainly lodge a dispute with your insurance company, alternatively with the ombud for long term insurance. His office can be contacted via their website, here: http://www.ombud.co.za/
They have a simple procedure whereby you can refer the dispute to the ombud. You need to obviously provide all of the relevant documentation, but then you sit with the ombud and await their determination!
If my answer hasn't answered all of your questions, please send me a REPLY with follow up questions so that I can continue to assist you in this same thread for as long as you need. Let me know if you need more advice - dont just rate my answer as "bad".
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Good luck and best regards,