Hi Martyn, you can't 'tune' via a scart connection, the signal is either there or it isn't. Tuning is only possible via an RF (aerial) connection.
I'm sure people more familiar with the LS 500 will respond, but my first thought is that as you're getting something through, then clearly the connection is OK - of sorts!
If you're not getting picture, only sound, are you using a fully wired scart cable? Some are only wired for audio.