Yellow leaves on my Winter Beauty Clematis....should I be concerned?

Good evening everyone. I noticed some yellow leaves on my fairly new clematis. I believe overwatering can cause this but with the weather being so hot recently I have watered it every day. The other cause could be a deficiency of some sort apparently. I've followed the planting instructions to the letter. It's in an 18" deep, lined, wooden container. There's grit on the top to contain the moisture. I've put so much effort into it, nurturing it from a tiny plant, I'll be devastated if it doesn't survive. Any help or advise will be much appreciated.

 

They aren't very well suited to containers as far as I know, but the container also seems to be only half full of soil. That means it doesn't have a lot of growing medium.
I presume there are plenty of drainage holes?

 

It's probably hungry and may be thirsty, altho this variety doesn't like to be too wet. It's also tender so, if you can't plant it in the ground in a sheltered spot, you'll need to find a much bigger container so its roots can spread and feed the plant which, when happy, grows to between 3 and 5 metres.

clemdetail -