He did a graphic novel - "The City" - which could be considered a sequel. Alas, he seem to think he is a "proper" author these days (I have heard he prefers the term "dreamweaver" ;-) ), and seems unlikely to return to his old ways. Although "Once" was Ok, I haven't truly enjoyed a James Herbert book for years, though I haven't been able to motivate myself to read "Nobody True" yet.
I know Domain and Lair follow on, but not sure which way round they go! had them in a double edition somewhere but, as usual, lent out never to be seen again!