Looks like there can't be a bet then, as I'd already said that I'd signed up to do the shorter course with the others, would walking up hills if the gearing stopped me getting up and i'll do it on the SS if the local practice runs don't go too badly.
Reason for attempting it on the DD is I guess the same reason as others have for attempting things that few people have done before ..... because it's a challenge. I could challenge myself to beat last year's time (but it's a different route so that wouldn't work). Or I could challenge myself to get around without walking the hills (I'm a realist and don't believe I can get up the 20%+ gradients without walking even on a geared bike - not only would I fail this challenge, but I'd be failing something many others on the day would be succeeding in). Or I could get a recumbent bike and do it on that (mmmm, tempting for next year!). But instead I've gone with the possibility of the SS.
The calculations and logic are pretty sound. The gearing of 42/16 should give me an average speed of 14.5 mph at cadence of 70 and 12.4 mph at cadence of 60. I'll be grinding up hills and freewheeling down the other side so I should be able to hit an average cadence of 60 I would think.
Ride starts at 8am and i aim to finish by 5 like last year - with 1 hour set aside for rests, this requires an average of 12.5 mph over the moving day - not too unrealistic.
As the day goes on I will get tired and the cadence will drop so I need to use the rests well. Last year we had that long stop at the cafe for about 30+ mins, this year I won't do that but instead use that time for shorter snack breaks of 10 minutes or so. This should help the legs stay warm and give me more frequent stops.
Doesn't logic on paper look so simple
But it is this logic why I know I can
complete the course on a SS if that's what I decide to do
I still expect to finish last, and have the company of the broom wagon for most of the course, but think about the feeling of crossing the line having ridden all that way in just one gear
. That is great news as it probably means that it will be a bit easier to qualify 
. Now I just need a completely windless race day so that the rear-disc-wheel-guys will have less of an advantage (I'm bringing a normal rear wheel).
