Hi Mark and Grumpy, that valve retainer should have pins in it and is pretty tricky to get back in. This is a good time to use a valve compressor tool if you have one. Although I can't see the pins from the angle of the photo . When you compress the retainer you should be able to see the pins and they will either fall out or they should be easy to remove hence you might need a little grease to hold them in place to re-install. Go slow with the valve compressor so you can see how the pins sit , sometimes the pins can stick and ride up with the retainer when compressing, so you may have to spray a tiny bit of degreaser.
