Assuming you have a 13/60:
A herald convertible is noisy enough at speed as standard - with the hood up, it's like sitting inside a snare drum at 60 - so I can appreciate that a noisy exhaust is a terrible idea for an 'upgrade'.
Standard single-pipe systems are still freely available and not particularly costly, and I expect someone would buy your current system from you if you eBay-ed it. Rimmers sell a mild steel standard kit for £80 which is what my 13/60 has, fitted by the last owner. The sound at low speeds is the typical 70's 4-cylinder rasp from times gone by - a friendly kind of note but still with more character and decibels than a modern car. At speed it's not quiet by any means but I don't find it gives me a headache or makes me angry / tired even over a long drive.
Rimmers (and the other big sellers) also have a stainless version for £150 but I have no idea about the acoustics - I assume it would be markedly quieter than your 'sport' system but sometimes the stainless ones do seem to start quiet and get noisier with age, at least I have found that with other cars. I won't be going down that route myself for as long as mild steel systems are still available for Heralds as I prefer the original sound-track (and smiling looks rather than cross ones from pedestrians).