Hevden
Well-Known Member
Although a Nightingale is not impossible, as there is always a chance of a vagrant bird, considering how far north you live and the fact the population and range of the Nightingale - traditionally restricted to the south-east of England - has drastically fallen in recent decades I feel it is prudent to ask whether you ruled out female Redstart - the two look reasonably alike!
Or considering it was singing maybe a garden warbler?