That didn't take long. Last night I pointed the domain name to the new Wordpress server. This morning I had 50 spam comments held for moderation. I was impressed the spammers had noticed the blog change so quickly, then I thought the robots probably change their behaviour depending on the comment page. Clever those spammers.
Still, there are plenty of solutions. Before I get into writing some unique code I found a simple plugin. If you want to comment you now have to answer a question. I've begun by asking you to type the number 6. You can answer with any of: 6, Six, SIX, six.
If the spammers decide it's worth adding that answer to their bots then I can change the question.