Letter for 3rd party claim

I have a third party insurance with dotsure since my car was pis up in October, this is after 4 years of uninterrupted comprehensive cover with them. On the 30th of Nov, 3 weeks after I changed my cover, I was involved in a car accident. My lawyers and the third party requested a letter from Dotsure indicating why I can't claim ffom them and why they are unable to submit the claim on my behalf to the other insurance. I have been sent from pillar to post to get the letter, was sent a blank document after being made to log a full claim that was totalky unecessary to start with since I am not claiming from dotsure. It's been 2 weeks now, still no letter and I cannot submit the claim to the other insurance without the requested letter.