As is said if you have a mortgage you must have buildings insurance. Why only subsidence? Fire is a far greater risk! I am sure your policy told you that you have to register a claim within so many days so they may be within your rights in not accepting your claim.
I had water damage in my attic bedroom last year but I couldn't claim because it was wear and tear, I didn't realise that I had a loose slate - fair enough, that's maintenance not an insured risk. The policy wording is usually very clear about what you should do.
Themas' advice is sound, but you need to be sure that you are in the right before pursuing it.