Hi,
Ask your solicitor what would need to be signed by them to allow you to claim against their professional liabilty insurance, to cover the loan amount if ever the bank decides you encumbered their mortgage without their "written" prior approval.
Just explain as a business operator, this is an insurance policy you need to take to ensure the secure running of your business, and that if they are happy with providing finance on particular terms (what you said before), then they should have no problem signing such a statutory declaration.
Again seek legal advice, ask them about disclosure of encumberance of mortgage, and how easy it is for lender to ask for full recall of finance.
Call it "sleep well at night insurance"
Just a thought
Michael G