Transfer share of mortgaged property to wife

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Hi

What are the tax implications if I increase my wife's share of a B2L property.

It was jointly purchased and we are both on the Mortage. It's held as joint tenants.

I am now a higher rate tax payer and my wife isn't. I would like to transfer 40% of my share to her. We have about £240000 Mortage outstanding. Would the stamp duty be on £96,000 (40% of the Mortage ) and would we need to pay the additional 3% stamp duty?

As the Mortage is in both our names would we escape for needing to pay additional stamp duty for the transfer.