Hi Mick 
In my limited experience, I used to follow the same gel/distance pattern in training and marathon. I did feel that by race day I wasn't feeling any particular boost after a gel - usually they take a couple of miles to kick in, so don't make the last one too late.
This past year I have weaned from them in training. I used to take them every 5 miles on a 18+ run, and started to reduce to every 7 miles until now I can get away with one gel in a 20 miler these days and don't feel any different, ie I'm not dying on my feet. Then just refuel immediately after running. I wanted to train myself to not need them and like you said hope that when I use them in a marathon they will give the required kick.
2 thoughts -
You will probably need to have at least 1 - 2 runs using your race pattern to see if you can tolerate them/that many.
I'm not sure how many you used when you were up to 16 or so last year, but I swear, you will be fed up with the taste of them, especially if you also take energy drinks (I don't). Personally I would go with every 5 miles max. Or 5, 10, 15, 19 and no later then 22.
Edit for clarification - 5 mile intervals in the race, nice round numbers. Your (generic "you"
) mind sometimes is not able to manage simple calculation when you're running for so long. In training, space them as far as you can get away with.
Edited: 10/03/2010 at 21:01