Possible yellowhammer baby attacked by magpie

We have had a little baby bird coming into our garden for about 2 weeks now. 
I’d never seen mum until today. 

Today I was looking out the kitchen window and noticed a magpie in the garden, and in the spot where this baby bird usually spends most the day - and then I noticed it was attacking the baby bird. 

I ran out and the magpie flew off. The baby tried to flap it wings but couldn’t fly and hid under a bush. 

I decided to leave it for a little bit as it was safe under the bush and I didn’t want to stress it out any more. A minute later a beautiful yellow bird came down and I realised it was mum as she was trying to care for it. 

I think its leg is broken Disappointed and it can’t fly. A little later that magpie came back. I shooed it off again and the baby bird went and hid inside a large hydrangea bush. 

If it is still alive in the morning, is there anything at all I can do to help it? I’ve grown quite attached to this little baby bird and I’ve never seen a bird like its mum before, looking it up I do think it appears to be a yellowhammer - not common around here I don’t think. 

if anyone has any advice I’d really appreciate it. 

Thank you