Powerflow do S/Steel. They have a series of franchises all over the place and they can make whatever you want (standard replica, leave out the centre box, a choice of tail-pipe designs etc.)
They'll be quite pricey but it'll be the last exhaust you buy for this car.
You only need the central and tail sections in S/S as the headers down to the cat hardly ever rusts (it gets so hot .. and any condensation will drain down into the cat section). Keep the cat for emissions reasons.
A mild steel one is cheaper and could be a good bet if you're only going to keep the car a couple more years. Standard AR back box is about £200 and the centre section not much less, so don't bother (you can get a stainless one for that much).
) do OE style ones for half those prices and they'll last 2-4 years easy.
I never tried the mega-cheap eBay ones but they're more likely to fail sooner (thinner/cheaper steel, not as well made, whatever).
Some aftermarket back boxes are stainless and some aren't. Prices tend to mirror genuine part or Powerflow prices. Beware stainless back boxes with mild steel tail pipes as it's then only as good as the weakest link (the joint between box and pipe).
Use new bolts and stuff too (your old ones will be knacked anyway) includig the rubber block that holds the silencer up at the back (they go hard and dry and it'll be a ***** to rehang your new pipe using that old block).