@blue peter – I assume they're the navy cords? Since the pants are textured, that makes it a good bit easier. I'd recommend a dark jacket – i.e. black or grey – and try to do something tonal with interesting textures. Trying to do something adhering to the "correct" CM rules of navy pants is going to be hard. Really the only options you'd have from that perspective are something in a pattern or in khaki. As for using a SS jacket – that might be fine. I often don't care for mixing seasons because the fabric weights feel too disjointed – but, that isn't to say it's unilaterally bad, just takes a keener sense to do well.