Obvious spoilers. Don't read this question unless you've read the book.
At the end of The Time Ships, our protagonist, is taken to the beginning of his current universe's time by the Universal Constructors.
When they break the T=0 barrier, they end up in a super-universe apparently created by the Watchers. Whom we have seen glimpses of throughout the story.
The Constructors aren't mentioned again.
One of Baxter's hallmarks is detailed explanation, but this little section before the end is a little light on his typical detail.
Perhaps this is meant to be an unimportant detail, secondary to the protagonist's goal of finally being reunited with Weena and attempting to, erm, fix the whole Morlock/Eloi situation.
Do the Constructors become the Watchers? If so, why do the Watchers resemble humans (in some bizarre twisted way), while the Constructors are basically rectangles of yocto-cilia.