Ride up through Donington Castle and get to Snelsmore... further afield there's the Ridgeway to the north, a favourite of mine is Wayfairer's Walk between Combe Gibbet and Kingsclere. None of this is classic singletrack though. I believe a Swindon MTB club has some sort of trail centre if you're prepared to travel a bit.
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Sounds good, give me a shout and we'll sort something out