Who is the housebuilder?
This large retaining wall at the rear of your property is clearly a material change that would your decision to buy.
But could it be that what you were shown on the plans when you reserved was different and was this still the case when you moved in?
Either way I do think you have a case for a breach of one or more requirements to the
Consumer Code for Home Builders 1.2 If you never received a copy of the Code.
1.5 Incorrect sales and marketing information.
2.1 Pre purchase information.
3.1 (b) Notification of changes.
Could it also be the case that the home at the rear which presumably has been built higher than the council planning approved plans show, should have either a retrospective planning application, which you could and should object to, or should be demolished as it does comply with the approved plans.
Finally is the retaining wall and fence your boundary responsibility?
From what you have said it could be that the fence may be able to pull the wall over (leverage) if the wind is strong enough in a particular direction.
You really need to write to the housebuilder and get your complaint on record.
Consider what outcome you want or need:
1) Your home bought off you by the builder.
2) The rear house knocked down and the retaining wall demolished.
3) A cash settlement to accept it "as is"
You will also need to have exhausted their customer service procedures before making a claim using the Code [CCHB.]