Thanks prospero, all complete now. Incredible how many words with thr in (there, three, other etc) plus, of course e.....
Idem for sd, said being immediate. Interesting that tl indicates two letters, not one.
I had visions of giving up on this one, and it would have been the first time, but perseverance and your very helpful observations have saved the day. Extremely difficult puzzle this time round - next month’s should be a walk-over by comparison.