On the advice of my mortgage broker that doesn't work with HSBC, he suggested I talked to them as I've been doing my banking with them for 10 years...
All was fine and dandy until they declined because I have a balance on 0% of their own credit card! I don't pay any interest and on time so it seems like such a silly thing to be declined on.
Other mortgage brokers have said I have to clear that CC, but that's about it, it won't affect my chances of getting one in principle.