fireback query - (what is it?)

Hi folks,
wonder if anyone can explain what a fireback is, and can I get one without having to buy a whole new firesuite. Here's the situation:

I have a chimney breast that had it's fireplace removed and a vent put in (previous house owner did it.) I want to put a fireplace back in, and so contacted a fitter who came and did a smoke test. He said it was fine to put a fire back in, just buy one and he would fit it.

Today I went to homebase and they had a nice firesuite (hearth, surround and gas fire) ,and it was on sale because there was no fireback. This was written on a sticker, as reason for sale. When I asked one of the shop assistants, what that meant, they didn't know.

What I need to know is can I buy the suite and buy the fireback separately? Is it needed? is it a standard size, or will I be stuck if I cannot find the one that goes with the fire? roughly how much would it cost (if it's in the hundreds, then I won't be making a saving, as the suite is down from £800 to £500.) is it a big job to fit one?

I would have asked the chap who was going to fit it, but he's on holiday, and by the time he's back, it might be sold.

any help would be greatly appreciated. PS. it is urgent, as the shop opens on monday and I need to get there early.
thanks.

If you need one, here's one.

looking at the link there seems to be only a few sizes. so I assume they are pretty standard. In theory, could any gas fire be fitted in one of these? they don't need to attched to the fire do they (I am worried that if the 2 need to be joined they might not line up.)
SHould they have a top bit with a hinged flap that could be opened and closed? It's just that when I was looking at a similar fire on a website, there seemed to be a flap above the coals. Is the flap necessary?