Sorry fir my ignorance. Is section A acting like retaining wall or is it B ? Or would all need to come down to the edge of property and new retaining wall built.
It's impossible to tell without looking what is underneath the fence.
There may be a retaining wall underneath the fence - I think that would be the most likely situation but, without physically checking it, it's impossible to tell.
If there is a wall underneath the fence then B would just be the equivalent of a raised bed in a garden which you can either keep or remove.
On the other hand, if there is nothing but soil underneath that fence then B is acting as the retaining wall, although it seems odd that it is so far into the garden.
Or would all need to come down to the edge of property and new retaining wall built.
You could do that if you really wanted to, but there is no actual need to do it.
I not sure how they would get mini digger in to back garden.
You can get very narrow diggers that are specially designed to fit through a normal doorway. See here for example, £80 to hire for the weekend:-
www.cjhire.co.uk/product/industrial/mini-excavators/doorway-mini-digger-1-ton/