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My electric shower keeps tripping, is it best to replace the full unit?

I have wall condensation ( black mould in one corner ) in one of the bedrooms. It's a 1930's solid wall house. A passive vent was installed, but no major change. Damp surveyor says it's only condensation and nothing serious, but to keep the room ventilated. I prefer heat recovery system compared to thermal lined wall paper. Has anyone used the Kair Heat Recovery Room Ventilator K-HRV150/12RH or another similar system with good results?

I had an emergency plumber last Saturday as water started pouring out of the vertical outlet into which the washing machine hose feeds. After rodding for 3 hours and using two packets of Drain Unblocker powder, there is still a persistent blockage somewhere in the pipework behind the unit/oven section up to the sink area. I rang local drainage experts but they won't touch it. I am not sure whether snaking is a good idea because I am in a first floor flat and don't want to end up liable for possible damage to the flat downstairs if there is pipe damage. I paid a lot of money for an emergency call out that lasted 3 hours with no result, which I felt was fairly excessive, but am unsure what to do now. I have done a partial refurb to my kitchen (new hob/oven) and am worried that removing the units to deal with the pipework might cause expensive damage that I am not sure I am covered for by insurance. I own the leasehold of the flat (not the freehold) and the Managing Company inform me that the pipework liability is mine. Can anyone possibly advise me as to a way forward? I was not planning on taking out/installing a new kitchen as I am nearly 70 and there is no lift in the building - just planning a quick make-over to sell. However I am wondering whether a kitchen refit might be the most cost-effective way forward? Any thoughts/advice from plumbing/kitchen experts, please? Thank you.