I THINK I've got it. (The one I heard anyway)
I think it might be a Hoopoe. Listening to the call on A Youtube video by Vicky Marble Run ASMR, it certainly sounds close to what I heard. It's similar on the RSPB site Though that one seems stuck to 4 repetitions and the one I heard did 2 or 3 at a time.
(I originally included URLs but it got flagged as spam - should be easy enough to track down though.)
From what I can find out, hoopoesare rare visitors on migration - April-May and August-September with a handful a year as far north as Shetland. So it's certainly a possibility. I've only seen them in the South of France before.
Afraid not! Glad you might have found yours though. Mine has more of a gap between calls and less of a hoot and more of a wow! I will try and record this year!!