Nightingales in the north east???

Hello, 

I’m not one for looking the bird life usually, but say a pair of small brown backed, cream fronted birds in the garden today that I’d not recognised before. 
I googled them and they look to be nightingales. Very distinct separation between the colour on the top & bottom of the bird. 
But Google says they shouldn’t be this far north. Could they be something else? 
There was also a robin in the garden & they were of similar size, if not a little smaller than the robin. 

I’m in Teesside. 

Thanks