Battle of the Giants! - S4-E14
Continuity mistake: When the Walmington platoon stop their van to open the gate to let the sheep out in attempt to hold up the Eastgate platoons van the Eastgate van drives through the gate into the field with Hodges on his motorbike following, but when the camera cuts back to the road the Eastgate van and the motorbike are on the road behind Walmington's van, the Eastgate van and the motorbike are next seen coming out of the field further up the road.
Battle of the Giants! - S4-E14
Continuity mistake: When Mainwaring and Square, driving the vans, are trying to push each others' van off the bridge, Hodges parks his motorbike at the end of the bridge behind the eastgate platoon's van. In all the following shots Hodges' motor bike has disappeared.
Answer: The show is a comedy, this was played for comedic effect and to show that in times of extreme fear, in this case about to be crushed by a steam train, the men had an adrenaline surge strong enough to pedal fast enough.