depends on budget, im saving up for a supersprint one but theres some on ebay cheap or i notice you're in norfolk so if budget doent matter go down to power and performance, theyll do a custom one for you.
Dont quote me on this but i dont think you can take the mk1 back box's off in one. You may need to cut it off as i think the standard ones are made in one bit.
However i did get a quote for a full sports system for £100 brand new off ebay.