I think the majority of PPs commenting haven't read ALL of OPs posts where she clarified yesterday evening that
. It wasn’t so much the fact that a note was sent, but more the way it was written - it came across as saying that DPs house would be more convenient for them, and it was a bit full-on. If they’d said “we like your house - if you ever wish to sell it we’d love to be considered” I think it would have come across better. But it went into details like the number of floors in their current house, how they’d like “less stairs”, how it would be better not to involve an estate agent, how “of course we’d pay you the market value” and that made it into something else.
It sounds too much in that letter they posted, as a simple note 'if they ever wanted to sell' as OP suggested , OP said they would have been fine. It was the tmi and contents of note that was too much.
OP also said she'd looked up Zoopla value of both properties and they are similiar values, so clearly the people who put note through door saying they wanted to 'downsize' (which usually means buy cheaper and smaller property to release some equity) are likely hoping to get it under its actual value.
'No need to involve EAs' might be their way of suggesting to an aged couple that there's 'no need to get it valued, we private buyers will tell you your properties value without it being on an open market'!! 
I get at least 6 letters through my door every year asking if I'm interested in selling my house, half are private buyers as well as usual EA letters due to living in desirable residential road with larger drives and gardens than usual.
Last year I had started to think about selling my house , so I did respond (with a might be considering potentially selling my house in near future) to a polite letter from private enquiry (as timing might have been right).....
You know what? ... the top amount they were hoping to buy my house for was £75k less than the lowest valuation (£110k less than top valuation) that I had 4 EAs do a few weeks later. (And that was on their 3rd call back to me (with their maximum potential offer which was £30k more than original suggestion they made!) . They knew the market and people hope for a bargain. I'm glad I thought "I'll get my house valued by a few EAs"