I have workmen in my house at the moment and I am so over the moon with the guy I picked. He has not patronised me one single time, listens to me when I explain the things I know about my (ancient) pipework and wiring, and genuinely consults me on decisions. At no point has he asked me where my husband is or told me 'what you wanna do is...'
This guy could burn my fucking house down and I'd still give him an exceptional review.
I should probably add that such systems here are all sealed pressurised systems - so nothing like the abortions with header tanks in the roof that are still used in a lot of systems in the UK. The hot water cylinders are all fed with full mains pressure so they are enamelled steel or stainless steel, not copper.
In the end the landlord didn't need persuading as the outer jacket of the cylinder sprung a leak and flooded the hall!
When we got the defective cylinder out, there were, as I suspected, pinhole leaks in the inner coil of the cylinder that allowed the mains pressure water in the outer part of the cylinder to pressurize the circulating water in the system.
A new cylinder fixed the problem completely!