Farquhar,
Welcome to the forum.
The numbers are Hayward. Same combo I have. S-244T sand filter with a series SP-711 variflow valve. As tonyl said the gasket is shot. Not unusual for the thing to lift off and get destroyed. Simple replacement. You will see widely varying ideas of how to replace it. Hayward recommends gluing it in place with crazy glue. (haywardnet.com). Think that's what I saw when I last looked a couple years ago. I'm trying silicone seal. Others on this forum say not to glue. Anyhow, that's your decision. After a time the initial gluing at the factory can fail and let the thing detach. Actually it is a simple replacement. Worst thing you can do is switch positions with the pump running or any flow through the valve. I learned the hard way.
Al