Location
Great Oxendon tunnel is located just off the A508, which is the main road that links Market Harborough with Northampton. Both entrances to the tunnel are on Brampton Valley way, so to go through the tunnel you actually need to be on the Brampton Valley way path.
There are many different ways to get on to the Brampton valley way path, there are numerous foot paths from local villages such as Arthingworth, Braybrook, Kelmarsh and Great Oxendon its self.
These footpaths and bridleways would be too hard to explain and confusing to understand if you had no knowledge of the local area.  
By clicking on the the directions page will find the easiest ways to get on to the Brampton valley way path reasonably close to the tunnel entrances from both the Northampton and Market Harborough sides.
If you have any problems please do not hesitate to ask.