Just a few thoughts to add to this: I have CI cover arranged as an optional benefit through a scheme sponsored by my employer, administered by Scottish Widows. In the small print is the key sentence 'To make a claim you must call Scottish Widows for a claim form ... within 21 days of being diagnosed for the first time' (my italics / bold formatting). That is awfully fast, although it does go on to say 'If you have not been able to notify us by then please still contact us as we may still be able to consider your claim.'
When I was diagnosed in the summer it all happened so fast, and it was just that I was sat at home feeling sorry for myself that I decided to look up to see what the claim process was. Thankfully it was only about 18 days after diagnosis, but it was getting close to the wire! Fortunately, the claim form was simple, and the copy of the letter from my consultant to my GP had all the info they needed, and they paid out.
As has been posted above, they will not pay out for prostate cancer if it is 'Histologically classified as having a Gleason score below 7 and TNM classification below T2bNOMO that are treated only by observation, surveillance and/or surgical biopsy' or 'Classified as TisNOMO or Prostatic intraepithelial neoplasia (PIN)'.
When they rang me to say that they would pay on the claim they added that whilst I could continue the CI cover, they would not pay out for anything that might be considered related to the original condition. In practice that makes it very unlikely that they would pay out if I was diagnosed with any other kind of cancer (they may take the view that one led to the other), and if I was to suffer a heart attack which might be linked to any medication I was taking as a result of my treatment for PC that too would probably be excluded. as a result I may not be continuing with the cover.
The key thing for me is therefore always read the small print, and don't waste any time in making a claim.