Alex Scarrow has announced that he is working on a tenth Timeriders novel. Nothing much is know about it at this point.
When talking about future projects on his website he said,
"TimeRiders10 (and more):
Yes....I am planning that. I intend to write that later this year. At the moment, no publisher has been contacted regarding publishing that. But that process will begin when, and only when, I've completed the book »