I have a birds nest at the back of my garden that is horizontal could it be a robin's nest.? the nest is made on one of the corners on a plant trellis.it looks like a sideways flower pot.
I don't know of any UK bird that would build a nest out in the open on the corner of a trellis.
Do you perhaps mean in the sideways flowerpot? Then it could very well be a Robin. If you can look inside the pot without disturbing the occupiers, robins make a tidy cupshaped nest .