I passed by the Somerton lock recently and there are signs of vehicular access via the margin of the fields, so I guess the owner may have an arrangement with the Farmer which would reduce the burden considerably.
The Kings Sutton cottage was for sale at 495k plus IIRC 3k per year for use of the access track. Note also Grade II listed building. Overall a hefty price tag.
Whilst many of the cottages on the Oxford are attractive, the access rights and cost are significant to say the least, however, it would be a great loss if they fall in to disrepair and are lost.
Edited to add extract from Kings Sutton (Travers Lock) sales blurb:-
Access
Tarvers Lock Cottage has a pedestrian right of way down the towpath along the Oxford Canal. Private access for vehicles and pedestrians is via a mile long track from the Aynho Road, Adderbury, which passes principally over New College Estate land. This access is for owners, visitors and guests only. A new 125 year lease at a rent of £3,000 per annum will be available to the purchaser from New College (subject to contract).