Thanks for the suggestion but this isn’t for riding, it’s for hiking, particularly in the Isle of Skye this March.
Thinking about it, and what I already have, I am thinking that, as a few have said above, a decent softshell may be what I need. I already have a Arc Teryx fleece, and various decent hoodies for insulating layers, and I already have a North Face hardshell as a wet weather layer. If I had a decent, windproof softshell, then I suspect I will spend most of my time with that as my top layer; only dragging out the hardshell in heavy rain.