In short, you are legally allowed piano tuners and other tradespeople into your home during tier 4 and tier 5 lockdowns.
In terms of exposure to Covid-19, Piano tuning is a relatively low risk profession. What social interaction I have had since march has been at a safe distance. I have good common sense and have been washing my hands religiously (with alcohol-based sanitizer), both in the car and when I return to my Sheffield home. I will happily wear a face mask on request.
A few links to keep your mind at rest:
Let’s support local piano tuners and tradespeople. But let’s do it while staying safe and obeying the lockdown rules.