Do you have the contract that governs the sale of the property? It is likely to have been in an agreement that you entered into with the Housing Association when you bought your share of the property. Invariably the agreement states what is to happen in the event of a sale and the fees payable and the basis of calculation of those fees. Please look at the prior paperwork and I am sure that you will find the answer. Happy to discuss further. Please leave a positive rating so that I am credited for my time.
This is stated as being "subject to contract" so wouldn't be legally binding unless there is a subsequent contract. You shouod query it with the housing association.
If you agreed to the sale having seen the memorandum beforehand, then I think you are too late to do anything about this. It should have been queried at the time. However, they still have to respond to your queries as a social housing organisation. If you aren't getting a response then you should be following their published complaints procedure.
Yes there is a housing ombudsman that you see details of online. The Ombudsman will consider complaints about the actions or omissions of a member, which is a Local Housing Authority, in connection with its housing activities so far as they relate to the provision or management of housing and of other members in respect of all their housing activities. The person complaining or on whose behalf a complaint is made must have been, in the Ombudsman’s opinion, adversely affected by those actions or omissions in respect of their application for or occupation of property. You have to have exhausted all other complaints procedures before going to the ombudsman.