You are unmarried therefore form 17 is completely irrelevant to your situation.
https://www.gov.uk/hmrc-internal-manual ... al/pim1030
However, there's a complication that in 2009 (I think) there was new legislation related to transfers of income streams that can mean that the transferor can remain taxed on the income.
My, non-expert, opinion is that if the income stream is not guaranteed then it looks more like a callable bond income stream which (probably) means that the taxable value of the transferred income is negligible.
e.g. You currently each own 50% beneficial interest (capital and income) via DoT (tenants in common). You transfer your share of the income to your partner. But the DoT (probably) has a clause that either party can trigger a sale of the property to extract their capital investment. As a result of this, the additional 50% income has negligible market value (How much would you pay for the income from an asset if someone else could unilaterally trigger its sale and receive the capital - hence my comment about callable bonds)
I do not believe you need a DoT to transfer the income. A signed statement to that effect is sufficient. (then again, I could be wrong...)
N.B. It is important that your partner actually gets the income. Safest way is probably to pay it into an account that only she controls. Even if she then pays it into a joint bank account it's then fairly obvious that it was her money and she decided what to do with it.
If the property is mortgaged then she will (I believe) only be able to claim mortgage interest tax relief on her share of the mortgage.
But wait for the experts to correct me!