We supplied and installed a conservatory. The owner of the property refused to pay and in court said the signature on the contract was not his, that he did not recognise the signature and that it was not the signature of any member of his family.
Can he still be liable for the debt, given that it has been installed on his property?
You should consider whether there might be a claim under the law of unjust enrichment / restitution. See:
I don't know much about this area of law I'm afraid.
You should of course take proper advice in relation to any actual or prospective litigation.