Hello,
The Pepper staff on the commercial side of the business have personal email addresses (which they openly give out on their business cards), so I laugh every time the Pepper staff on the retail side of the business tell me that they don't have email etc.
It's possible that the title deeds are held by a third party, who did the original legal work when the mortgage was created and the homeloan advanced. Contact the solicitor's firm that did your side of the legal work when you took out the loan, and ask who did the legal work for the lender at the time. Also, ask if they can assist you by contacting the solicitors who acted for the lender at the time to "give them a nudge". The solicitor who acted for you might be willing to do that if they feel that there is other potential business there for them (you may need them to store your title deeds in the future, or you may need a will prepared etc.) Failing that, you could try calling them yourself and asking if Pepper have contacted them, if they have returned the title documents to Pepper etc.
Obviously, continue to put pressure on Pepper through their complaints procedure. You must follow this procedure and get a final response from them, before you can take the complaint to the Financial Services Ombudsman.