I don't know where the Viper is, but Writtle is within riding distance from the western fringes ... unfortunately I'm in Reading tomorrow and wont be able to be back at a reasonable time.
However, next week is scheduled to be Gt Waltham which is again very reachable, so please don't change that. Most definitely don't change the 22nd March, with Hatfield Broad Oak being perhaps the best destination MEMWNS has ever visited.