Vendor didn't disclose major water ingress in semi-basement, what are our rights?

My wife and I purchased our first family home about six months ago. The property was previously owned by a Trust, with the settlor (an elderly woman) and her lawyer acting as trustees.

In early October, during heavy rain, we discovered significant water ingress in our semi-basement/garage. There was approximately 100 liters, which I removed using a wet vacuum. The worst-affected area was a space marketed as a “hobby room.” This room, located in the basement, is partially built into the ground and, while not technically classed as habitable space, features wall lining, lighting, and electricity, though it has an unfinished concrete floor. The water appears to be seeping through the concrete.

This issue was never disclosed during the sale process. It was also missed in the builder’s report, likely because the room was filled with old tables and kitchen benches which we regrettably agreed to accept as chattels when signing the sale and purchase agreement. After removing these items to clean up the flooding, we discovered clear evidence that this has been an ongoing issue, including staining on the concrete floor and mould on the bottoms of the benches. To us, it makes sense why the builder could not see the issue because the evidence was obscured by the amount of junk left in the room.

We raised the issue with the lawyer/Trustee, who provided advice from the settlor’s son-in-law stating basically:

1. The problem was known.
2. In their opinion, it is typical for this style of house (1960s brick and roughcast).
3. They do not consider it a major concern.

To us, this is a serious issue. The affected room sits directly beneath our three-year-old daughter’s bedroom, and we noticed increased condensation in her room during this period. It also means the space cannot be used as intended, as we cannot safely store valuable items there. We are currently awaiting a quote from a waterproofing specialist, but expect the remediation to be costly.

I work in an industry related to property sales and understand that vendors are required to disclose latent defects, particularly those involving weather-tightness. I also know that we will need to disclose this issue if we sell the property in the future.

What are our rights in this situation, and how should we approach getting the vendor to accept liability?

Thank you for your help.