We all know what prompted this statement,and of course it's wrong to collude with a tradesperson in order for him or her to evade tax. On a technical level however, it's perfectly legal, and not at all morally wrong to pay a plumber(or anyone else) with cash.
I think the people of this country are heartily fed up with the little lectures on morality which are fast becoming a trade mark of this government. It's one thing to impose economic constraints - we can all recognise the need for that - but it's a totally different and distasteful thing to practice this constant finger wagging, as if we're all a bunch of naughty children, caught with our hands in the cookie jar.
Human nature being what it is, people will continue to behave according to their own set of moral principles, regardless of what a rather silly Minister might say.