Alan, I think its a good idea as long as you don't go flat out for a great time at the Potteries. I did a couple of marathons 4 and 7 weeks before IM Switzerland last year, and as you say, just used them as long training runs. Even just adding ten minutes to your marathon time gives you nearly half a minute a mile extra which should feel pretty comfortable at that stage of your training.
Ignore the old adage that you should only do two marathons a year. Thats for sensible people, not Ironmen (or Ironwomen).