I find it reassuring that your buyer's solicitor is doing such a professional and thorough job.
Or perhaps, is he just lazy expecting you to find all of this for him?
That said, it should be your solicitor he is dealing with and/or your solicitor putting together the paperwork required.
Building Regulation final certificate would have been passed by whoever inspected for Building Regulation compliance
I suspect given it is Barratt, that this was also performed by the NHBC. Barratt will have copies of the CML completion certificate too. After 8 years I think everyone could assume the home was actually fully completed!
Planning permission is available from the local authority.
Section 38 agreement is for the adoption of roads. Quite why the solicitor on the one hand asks for this and on the other, asks for details of maintenance assuming non adoption means never adopted.
You perhaps need to be aware that after nearing 8 years, the roads are still not taken over by the council, it may cause a problem with your sale. Furthermore, by now the council should have used the bond and completed whatever works required to bring the road to adoptable standards and taken it over.
Section 106 agreement is for provisions in return for planning approval such as a financial payment to the council and affordable housing provision on the development, public open spaces.
Section 104 agreement is for adoption of sewers by the water authority. No doubt this is also under dispute and may even be the cause of the roads not being adopted yet.