Thanks for your observations and suggestions.
This is however doing my head in here.
I took a news post (using copy and paste) and posted it twice, and the "problem" was there and I presume for the reasons you state (ie about the "<p>s").
I then typed two short posts to see how they came out (line-height: 1.5
and it was fine in the full article and also as an excerpt.
I do not understand why this is a problem now every time I want to copy and paste from an existing article, news report, web page or whatever and I am getting height issues on the excerpts - this was NOT happening a couple of days ago when it was behaving. I still am wondering if something else is contributing to this?