After leaving #5: Flintwinch, take the path that heads downhill to the west and follow it until the path turns downhill. Follow the path all the way down to Ashford Farm where you leave Ashford Hangars Nature Reserve.
Where you rejoin the road, turn left and pass the turkeys to your left and above as you walk eastward down the sunken lane.
You are looking for a six inch long by two inch diameter black tube.
Dont forget to note down the letter and number combination from the cache so you can find the bonus caches later.
The last cache from which to collect a letter and number combinaton is Steep Marsh #7: Oliver Cromwell (GC1VQZY)