Mortgage Offer and further credit checks

I've had my mortgage offer since mid December and are due to exchange contracts within a couple of weeks.

My OH and I have decided to combine our finances, so we thought about adding him to my HSBC Advance account to create a joint account. Will HSBC credit check either of us and if so does anyone know if it will be a soft credit check?

I'm worried we could be credit checked by our mortgage provider prior to completion, and I don't want to jepordise anything.

Thanks in advance.