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Update. She spoke to a plumber, who had done previous work on the premises, for an opinion. He has just been and he mentioned the combi, and she said she wasn't keen. All eggs in one basket etc..
Shows that I misunderstand; I thought the important drop was from the hot cylinder to the taps, he was saying it was from the header to the cylinder, but looking again on the Net it now seems the important drop is from the cold header tank to the taps. Arrhhhh.
He mentioned that the flexible connections at the sink and basin restricted flow, and suggested the main problem is the kitchen sink and a new tap might help a bit. We're unhappy with the existing tap anyway so no issue with replacing it.
No suggestion of a pump though, and I wasn't going to suggest to grandma how to suck eggs. Puzzled it wasn't considered though.
I've just been back into the loft; yes the hot pipe does have a tap in line. Are they as bad as isolation valves ? This was one of the type with a flower shaped 'knob'. It was fully open. I was wondering if it was causing issues. Would it help if it was replaced (I'm assuming it's there for a reason) with one of the bar handle types ?