The Journey Planner suggests the 1637 from Oxford to arrive into MKM» at 1803. Though if you were already at Oxford I can see why you would want to keep going to minimise the risk of the later trains being delayed and thus missing the connection.
We tend not to risk the 16:30 off Paddington / 17:13 (?) at Didcot if we can help it. There are connection that usually make even if they're not supposed to, and connections which are supposed to make (official) but fail far too often. The 16:30 from Paddington, with a 6 minute change at Chippenham, is in the latter category. "First available train" rule applies!
