Hmmm... Spain or Watford... you have a tough choice there
I've run the Majorca marathon and it was a club trip. We organised it through Running Crazy and it all went well.
I've also run Copenhagen marathon and did that independently, flights, hotel etc
Both trips were great but personally I would only go with a tour operator if I was going with a group, such as my running club. But then I guess if you're going to be travelling on your own it might be good to sign up with RC and socialise with the other peeps who will be running.