Very helpful post McGlashan
Your welcome. The first thing you said was you are exchanging contracts and I missed that
In an ideal world the seller rectifies the issue and then you proceed like normal. See what your property solicitor advises. The catch being it's not some free quotation jobby. Diagnosing where the fault lies will involve some expertise so don't expect that to be free (the underfloor stuff is still a niche subject). Afterwards it's a straight forward case of sourcing and installing the replacement part.
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