Quite wrong really. From sheffield park you are within an hour of the south downs way, with some decent cross country routes, and if you can get into brighton, stanmer park and wild park way, very decent singletrack woodland decents.
If you have the car, definitely head to Friston forest, disappointment would not be an option!! Also have a look at http://www.deersleapbikes.com/about-deers-leap-park/park-map.aspx – which has some fairly decent north shore in the central woodland.
And what he said ^^ about Ashdown forest. I drive past there, salivating, looking at the gorgeous singletrack and just wishing those stupid people would come to their senses. Get this. HORSES are allowed but BIKES are not allowed due to them damaging the environment!!!!!!!!!!!!!!!!!!!!!!!!!!!! Most ridiculously stupid thing ever??????