Not only are these guys breaking the law but they are part of an entirely unregulated trade, which inevitably leaves people out of pocket, with huge problems needing to be put right, but more worryingly, with potentially lethal consequences.
There is nothing wrong with paying cash. For the most part it is these days much easier to use cards or cheques, but for those a bit like me who would prefer not to give banks an opportunity to skim off the top cash is fine. What is wrong though is people knowingly asking for a tax-free job, and paying cash folding to keep things off the books because what they are doing is inciting someone albeit a willing party into committing tax fraud.
I realise that at the moment money is tight everywhere, but what is far more important than even the Revenue's £2 billion estimated annual loss, is safety. The reason people like all my engineers train for a minimum of 3 years as an apprentice, plus addend college a day a week at the same time is because mistakes made by unqualified trademen, who are typically the ones working in the black economy, cost lives! For this reason in many European countries not only does the tax dodging builder, electrician or plumber risk prosecution, but a person found guilty of buying black labour is up for a 3000 Euro fine too. This works well to discourage the duel problems of tax dodging and dangerous workmanship, and I'm convinced if we took this idea on board and added a US style licensing system for all trades the days of the dodgy tradesman would be well and truly numbered. All we need is a Gas Safe (formerly Corgi) registration system where the public know they must ask for a registration number, and fully recognise the risks they are taking if they use an unlicensed tradesman.