Thanks for the above, but I'm a real novice at this stuff. Do you mean they gave you another dish, as well as a new sky+ box? And how do you use the old spare ordinary box, is it plugged into another tv and do you actually receive sky through this as well?
Not always, the LNB is different (so that has to be changed) so to be able to use Sky+ to view one channel and view another you need two inputs, hence the new LNB (the LNB is the bit that sits in the middle of your dish and has the cable to your Sky box from it), anyway you don't have to pay for that as it's in the installation fee, which can be free.
And more to the point, do you have to pay more per month for using the new sky+ box?
Yes, £10 unless you subscribe to 2 premium channels then it is free.
Some useful info
here.