I'm replying to Highly Eccentric's comment on the previous post in a new post, as I wanted to show some pictures. This is Edward, Prince of Wales, from Bruges' Garter Book of c. 1430, with a much older version of the ties holding the mantle:
And then by Charles I's time, they had become so long (especially on a young man: here he is as Duke of York) they had to be looped up into his sword belt:
At the Restoration, Charles II regularised the Garter "underhabits" with "the old trunkhose" of cloth of silver, which persisted at least until Edward VIII's time (shown here as Prince of Wales, complete with enormous ties):
And yes, you are right that the blue ribbon is worn when the full robes aren't being worn. The image of St George on a ribbon is called "the lesser George", and replaces the big chain, or collar, with the little model of George killing the dragon you can see hanging on William's chest in the previous post. By 1508, it was recognised that this collar was to be worn only on feastdays, and "on the other days the image of St George shall be worn at the end of a little gold chain, or in time of war; sickness or on a long journey, at the end of a silk lace or ribbon." In the early seventeenth century, it became customary to put it over the left shoulder and under the right armpit, "for conveniency of riding or action" in Ashmole's words. You sometimes see this in portraits that emphasise the military accomplishments of the knights.
As ever, I'm indebted to Peter Begent and Hubert Chesshyre's authoritative book, The Most Noble Order of the Garter: 600 Years, published by Spink in 1999, for many of these details.