David and Peter in Not Scarlet But Gold, I think is their first kiss, although you could see it coming from Neglected Mountain.
You then get Jenny and Tom, which is rather sweet, I think Jenny kisses him on the cheek quite early on, perhaps in Lone Pine Five when he buys the spoon, but they only really get sorted out in Man With Three Fingers.
Lastly there's Penny and Jon. I was never convinced by that. Penny seems too much of a flirt to stick to her rather boring cousin when she meets other boys after being at her all girls' school.
Don't really count Harriet and Kevin as a couple, although I think the twins do. Seems more a friendship to me.
You can tell I've read them a few times. It's interesting because people have said that modern children find old books too boring with descriptions and just want snappy dialogue. But ds, whose concentration is on the low for a 9yo boy, absolutely loves them and will reread them again and again.
Lucky you writing to him. I think he died in 1982, which was a few years before I read them. I think he aimed to reply to all letters. have you still got it?