SDLT on transfer of mortgaged property?

I understand that when beneficial interest in property is transferred between spouses the amount of mortgage liability assumed by the transferee is regarded as consideration and SDLT is payable on any amount over the threshold of £125000, but can anyone tell me what happens when:
• Spouses hold the property as tenants in common
• the mortgage is in their joint names with joint and several liability i.e. each party is liable for the whole debt
• the share of ownership is varied to optimise the use of the basic rate tax band

I have thought of two possible answers but is either of them correct?

1. there is no consideration and hence no SDLT payable as the transferee has not assumed any additional liability in relation to the mortgage
2. HMRC would argue that the transferee has effectively taken over responsibility for the share of the mortgage corresponding to the share of ownership transferred i.e. ignoring the joint and several provision

Would the answer be any different if one spouse takes 100% of the beneficial ownership, having previously owned 50%?

Many thanks

David