"Oh Shit" moments...

well okay ill describe the road, there's a T junction with traffic lights on each side and a tunnel under for the canal, so when its green you start on a hill, by the time i'm in the pedals and ready to go it goes downhill, then its a slight downhill then flat for about half a mile, i was on a 46:18 spinning pretty fast in the drops

so for about a quarter of a mile i can go pretty speedy

With 700x23c tyres your 46:18 is about a 67" gear. Spinning at 100rpm, you are doing 20mph, you need to spin at 120rpm to get to 24mph.