The wheel slip on the very steep hill was in dry conditions. However the road had been polished smooth and there was a fair amount of dust on the road from the fields. If my aim had been just to get up the hill then I would have tried to ride further up, but as I still had 20 miles to go I didn't want to fall over and break something on the bike.
Next time I must take a camera - as the ride far more scenic on a Cruzbike - In fact I wish I'd gone slower at some points just to take in the views!!
... my next planned big ride is a 130 miles over night trip with ultra light camping gear. Definitely taking a camera with me that time!!