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Am I right in thinking it is acceptable to do away with the middle exhaust box and just have the end box on an otherwise standard exhaust system and standard engine? That is to say it will not result in excessive noise, tuning issues, an MOT failure or similar. supercass

I think it depends on the system. My 4 pot has a tubular manifold and stainless system (Rimmers I think) with centre and back box and is reasonably rorty (I have even been asked if it is a V8 more than once). It does not have a problem with the tuning or MOT. Never tried it without the centre box but if it was much noisier I think it would be too much to drive it far.

Many years ago I did that with my Mini.
It was very loud as a result

Albeit on a V8, I had a single box single pipe system and it was fairly loud.

The worst part was the incessant engine drone or boom at cruising speeds. I'd want to turn off the motorway or dual carriageway after 20 minutes as it drove me nuts. Noisy was nice under acceleration on B roads but that constant noise on the motorway

I replaced it with a two box single pipe system, still throaty under acceleration but the booming disappeared at constant motorway speeds. The first box should act as a resonator taking away those frequencies.

If you were changing the exhaust to a free flow sports system and the air filter to a K & N type, you'd probably have to change the SU needles to suit.

At the end of the day it's down to your personal preference and relationship with your neighbours.

The first box is a resonator, the second box a muffler. One takes out the higher frequency sounds the latter lower frequency sounds. I also had muffled Thrush chrome tips (no longer available) but lost those at Monterey last year. The car sounded like it gained 20 HP but still not bad. Stock resonator, Magnaflo SS muffler. Wouldn't want it any louder. As mentioned, one thing at lower speed on rural rods, another on the highway. I wouldn't take it out unless there was a good reason to.