I don't want to see a completely cashless society. It has massive implications for control of citizens and invasion of privacy.
I don't think that preferring cash is a sign of anything nefarious - it saves on card fees for a start - but offering two separate prices for the same job (one for cheque/transfer and one for used fivers) is different. Why would someone do that unless they were hiding money from the taxman?
I can understand a window cleaner preferring cash, as it might be easier to have one system instead of more, but they are dealing in lots of small amounts. Rewiring a house or fitting a bathroom will cost thousands, and the difference between paying tax and not is substantial (and that's before you include VAT). Anyone complying with this will have to go to the bank several times to get enough money, so it will be inconvenient at best. They must know what is happening, and be colluding in it. If they weren't aware, surely they would ask why they couldn't pay another way?