Pigeon Egg

Hi, looking for some advice on a suspected pigeon egg. Our cat came over the fence yesterday afternoon with a “gift” for us which turned out to be an egg. My partner rescued the egg from our cat which was still warm. The problem is that our garden is literally surrounded by other gardens that we can’t access so we have no idea where the egg has came from and can’t just go and have a look to try and return it to its nest. 

My partner did a bit of research and candled the egg which revealed what we think is veins/vessels and there is a mass inside the egg. So, we currently have it in a plastic box (the box is open at the top), nestled on a towel with a heat lamp and the blind shut - this is in the spare room so away from the cat. We’ve checked again today and the mass appears to be moving ever so slightly and the veins/vessels are still visible. We were inclined to throw it out but because it was still warm we wanted to try and do something to help it. 

Can anybody give any advice on what we should do with the egg or organisations that would take it and give it a chance? 

Any help is much appreciated! Thanks!