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This is around my regular training route and is pretty flat all the way. I've not done it myself but I ran into last year's race (almost literally) while I was out for my long Sunday run. It's all road except, I think, for the finish.
The Bowood 10k the week following is a very different affair. It's got quite a bit of off road in it and is considerably more "lumpy". Even the organisers describe it as hilly.
It depends what you'd perfer really. The Clock Challenge is road, flatter and by default a quicker run. It is still in pretty countryside but not as dramatic as Bowood's scenery - but you pay for the scenery with a hillier, multi-terrain and inevitably slower race.
Of course - you could do both??