I found this: https://tools.oratory.com/altcodes.html , which is the sort of thing I was after.
Ah, was that all you were looking for? That's been a capability since Win3. If that's all you wanted, you - again - need to look no further than windows, although this time it is hidden. A utility that has been there - again since Win3 - is Charmap
WinKey+R, type charmap into the box, press enter. Hey presto, you can select and copy or look up the compose sequence.
Alternatively, as before, you can start typing "charmap" at the start prompt, and the application will appear and you can launch or drag it onto your desktop for a shortcut.
(and yes there are compose sequences as well, quickest ones are with the AltGr key, eg € where the $ is)