There are days when I want to carry a few extra clothes on my commute e.g. if I’m running, going out after work etc but struggling to fit it all in. I don’t want a bigger rucksack and I’m not a fan of panniers. I’m thinking I could wear a smaller messenger back across my chest like a bandolier. Does anyone do this and how do you find it? Any recommendations on which bags are good for this?

You could do, but it’ll be awful. Just get a slightly bigger rucsac.

I generally use a post bag day-to-day, and use a rucksack as an overflow on the days that I need to lug something extra.

Keeps the weight off your back.