Adele
Sometimes contacting these helplines can prove to be a painful experience.
What I would suggest that you do is phone the Halifax Home Insurance complaints line on 0845 602 7377.
If you provide them with the previous owners Insurance details or their name/address and then explain the situation, they should be able to help you. What I would expect them to do is ask for you to provide a copy of the structural engineers report confirming the subsidence issues and when the problem started. The Halifax should then arrange with you to issue you with a policy and then to register the claim. The Halifax will then appoint their own loss adjuster to handle the claim.
If you point to the Association of British Insurers Subsidence agreement, they should be aware of this.
If the Halifax complaints team prove to be just as unhelpful, ask to speak to the customer service or custromer relations manager.
Huckster