Visitor staying with relatives

Just had [another] brain twitch after reading through about all the documentation recommended for visitors to avoid hassles with IO.  So I plan on getting a letter from work giving my expected return dates and that kind of thing.
I will probably have to spend more and more time with my ailing husband and daughter who are in the UK.  We own our home in Leeds but its DD who is the sickest bunny and she lives in a small flat in London.  There are only two bedrooms so when her BF, DH and I are all there it is pretty full.  If DS comes as well he sleeps on the sofa bed.  Should I technically get "permission" from the landlady (who lives in San Francisco)?  She knows the situation and is sympathetic.  Would they ever check up on our over-crowding?  They've never asked before.