Chrissie,
As you probably know, the Sky and Freesat satellites are rumoured to be close enough to one another that you can use the same dish without any reallignment.
A colleague of mine claims to have achieved seamless co-existence of Sky and Freesat simply by virtue of the fact that he had a spare downlead cable from an existing Sky dish which he was able to connect to a Freesat decoder. He tells me that some Sky installations will have spare leads and some won't, it all depends on how it was installed. As far as I know, he just had to buy the Freesat box and plug it in. Unfortunately I don't have either Sky or Freesat hence I can't put any of it to the test for you, and I'll deny all responsibility if my colleague turns out to be misleading me, though he's normally trustworthy
If you want HD, you'll need to connect the Freesat to your TV using an HDMI cable, so you'd need to make sure your TV has an unused HDMI connector to plug it into. I'd be surprised if that was a problem with a newish Panasonic, but you'd want to check. You can spend a lot of money on HDMI cables, though I've always bought the cheapest ones I could find and had no regrets.
Also, I assume you've noticed that some support HD and some don't. Obviously you pay more for the HD boxes.
- SLM