Definitely not a great tit - but what is it, pls??!

Sorry for the short clip, I cannot record it as it only does 3-5 low “whoop” sounds each time and it can be several minutes between each call again! This is the only time I have heard it while walking, it is usually while I have my skylight open, but does not sound like it is too close by. I live in rural Staffordshire and have been trying to determine what it is for over a year - I never noticed it before I started working from home. Please, someone, put me out of my misery?!