Thanks for the reply. Its for an engineer post. I looked at the ACPO for security systems and in the Disclosure of Convictions section states that 'spent convictions warnings, cautions etc cannot be considered'. It also states the police check will be limited to a PNC check against criminal convictions only. Therefore I would assume it will be a basic check only and hopefully nothing will show up regarding his uncle.
And yes it would surely be illegal for them to refuse the position because of someone else.