Curse of Fenric makes a fine followup to Ghost Light. It's got vampires, Ragnarok, game theory, cryptography, World War II, sea monsters, time paradoxes, causality loops, and ancient exiled evil.
Most interesting is the "Battle not with monsters lest you become one" theme. I can't recall an American children's show ever uttering the word "Dresden"; hell, I'm hard-pressed even to think of one that actually showed a swastika. Acknowledging that the Allies were far from innocent in the war is powerful stuff for -- what was the expression Sharkey used? -- goofball foreign children's television.
The plot twists are obvious, the characters are simplistic, and all in all it's a lot easier to follow than Ghost Light. But simplicity aside, the characterization is excellent, particularly from the Doctor and Ace during the climax.
All in all, a great mix of elements, well-written and well-acted. I think I'm going to have to agree with the fanboys: it's a pity the show was cancelled just when it was starting to turn around.