Glad its getting fixed, but bottom line is the only reason the tenant didn’t have hot water is because they were refusing to use it even though the plumber said it was safe.
Sounds like it just needed a new part anyway.
Still alls well that ends well.
He’s here now. I’ve only been using the hot water for very short periods the past 5 weeks. I have health anxiety, and the prospect of a huge explosion definitely makes it worse! 
New boiler in. Took him over 6 hours. An “ideal” Logic max combi. And a remote control with it! Not sure how to use it though, but I’ll get the hang of it.