Hi Bernie. Firstly can I say well done on your progress so far it's looking good and your hard work has paid off, and despite the patches you have a lawn .
I seeded a new lawn about 8 years ago, back breaking but satisfying.
If it were mine I would maybe try to ignore the patches for now and concentrate on the vigorous growth you have. If it's ready for it's first cut (on high) it will encourage it to grow thicker and stronger. The patches could then be re-seeded in the autumn.
Alternatively if you can't put up with the bare bits, is it possible to "fence it off" a bit with some canes and string to mark the area if you want to try seeding again now. Then you can mow the other bit and not disturb the seed bed.
Hard to tell from the photo but is it just the one area where it was washed away that's patchy?
I'm no lawn expert (though I have the Hessayon book)
Good luck ?