Hi! This thread is a bit old now but I'm glad I found it. I've been stressing out about something very similar...and this thread is helpful.
I recently received a letter with similar wording to "protect the ground rent interests" and mentioned adding a non-invalidation clause. We received the letter after we had already made arrangement for home insurance (which we needed for the exchange). Out solicitors have no idea what they're talking about, and our insurance company doesn't know either.
Did you manage to resolve this and figure out what was needed? interestingly our letter also directs us to their own insurance policies which they are trying to sell....
Thanks