From The Telegraph (online):

In his letter to the Prime Minister, Sir Laurie Magnus, the independent adviser on ministerial interests, said: “It is highly unfortunate [...] that Ms Rayner failed to pay the correct rate of SDLT on this purchase, particularly given her status and responsibilities as the Secretary of State for Housing, Communities and Local Government and as Deputy Prime Minister.

“She believed that she relied on the legal advice she had received, but unfortunately did not heed the caution contained within it, which acknowledged that it did not constitute expert tax advice and which suggested that expert advice be sought.”