IT IS EASY AND CHEAP
To get both analogue and digital terestrial television through a STB all you need to do is connect the STB up like normal then connect a TV to Video Lead (Plug to Socket) into the RF Out or on some boxes its called a loop through, but it should be next to the RF IN.
Plug the other end of the TV to Video lead (Plug to socket) into your TVS arial socket and away you go youve got both analogue and digital televison.
This is handy as when transmitter dosent put a great signal maybee in adverse wether conditions you can still pick up analogue televison without messing around with the STB.
It is also quite handy if you can recieve all channels but maybee not one of the original analogue channels, it may sound abit awkward but you just have to switch between analogue and DTT on your television remote.
I purchased my Lead from my local shop Around a pound , It was a Connect It 4ETVLH and cost only £1.20 for a 4mtr lead.
Just to make sure you buy the corect one if it is the Connect it lead you are going to buy the barcode on the back reads: 5024996025750.
I would suggerst taking a look around your local pound stores one of them should have one in stock.