I am almost ready to move in to a new build house but 1/3 of the already small garden is unsuable due to the steep decline which leads to a manhole. I can't build up because of going above the neighbours damp course on their garage. I am worried the ditch will become a swamp during wet weather. I exchanged contracts in July and I am due to complete in a few weeks.

As a first time buyer I have no money for an expensive garden to be landscaped. Do you need to do anything about it before I complete or should I wait until I move in?
The house was effectively bought off plan as it was not there when I had to exchange.

put a clause in the purchace agreement stating that this will be taken care of if it becomes a problem.

You can get in touch with the environment agency (?) who will tell you about flooding in the area. You can find it on the internet. This is a free service but it might take a week or so.