I think you have to look closely at the definitions in the transfer under the Right to Buy scheme as to whether references to the Council could include its successors in title. It might be that you can argue that it is not for the HA to give the consent but the Council. Certainly that is what happens in my area where a similar thing has happened. Consent is sought from the Council in such cases where they originally sold the houses (and they charge £10-20K for it) even though the housing stock itself now belongs to a HA. However in your case if you tried that I suspect that the HA would put the Council up to refusing so it might not take you any further.
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