A Doctor Who Missing Adventure novel, featuring the First Doctor, Ian, Barbara, Vicki and the Gunpowder Plot. I think this is the first Who book I have actually given up on. I found the first hundred pages stylistically dull, historically stupid (James I's father was not blown up at Bannockburn, the Doctor is rather unlikely to have tried staying at monasteries in England in 1605) and really offensively anti-Catholic. I skimmed a couple of online reviews of the whole thing to see if it might be worth persevering, but I rather got the impression that it just gets stupider and more annoying. I am glad to say that Roberts' other efforts at this period (DWM comic strip "A Groatsworth of Wit" and TV story "The Shakespeare Code") are much more successful.
Edited to add, March 2012: I have given this another try, and I withdraw the judgement that the book itself is anti-Catholic, but stand by the rest.