Thank you 2cv & Robbo for your simple (but to me brilliant) answers!
Phew! It hadn't occurred to me that the digital signal from terrestial broadcasters was different to the digital signal from satellites!
I do have a Digital Satellite Receiver that came with the Dish: it's a
Skyline DSR 4000. However (sigh) the Remote Control is missing. I had hoped that the Television's internal Digital Receiver would mean that the external Digital Receiver was not needed but the Dish could be used. Ah well...
Thank you again for your replies, I'll start again with the external Digital Receiver (I'll either get another Remote Control or try to operate it manually) and let you know how I get on.