As a last resort, smile sweetly, and make noises about opening an account in another bank. If you have a lot of dosh, that may make HSBC treat you differently.
My local RBS branch stamped & signed my printouts with no questions. I just explained why I needed them, and they were very helpful. This was in 2011, so branch policies may have changed. I found that banks in smaller towns are more helpful, likely because staff recognize you when you come in. Having a healthy balance will also help.