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Under the Right to Buy legislation, the Deeds to the property will have to be in your Mother's name alone, and if the Deeds were transferred into your joint names during the discount period, then this would trigger off the need to repay part/all of the discount depending on when the Deeds were transferred.
What normally therefore happens is that a separate Declaration of Trust is drawn up and which is signed by you and your Mother confirming that she holds 2/3 of the property on trust for you. To protect your financial interest in the property, a Restriction can then be registered on the Deeds, which will basically prevent your Mother from selling the property without your consent. Your Mother's Conveyancing Solicitor will be able to help with all the above.
I hope this assists you and answers your question.