Sorry to sound patronising cos I don't mean to be, but could it have been a dove or a pigeon ? I know some people have been fooled by those birds before? It is too early really.
I think I may have found the solution. We have a large number of blackbirds in the front garden and I suspect one of them is mimicking a cuckoo. We had one in the garden in Cumbria before we moved that could mimic my mobile phone ringtone. Nearly drove me mad
It's starlings that mimic rather than blackbirds. I once had one that mimicked me calling my cat in all the time. Even the neighbours thought it was me..