In reply to Tim Davies:
There may be excellent clues on the ground as to whether a RoW has been correctly classified as a bridleway or a footpath, eg. is it wide enough for a pony and cart, or is it too narrow for a packhorse to pass another....etc.?
As others have said, the county's definitive map should be the first (and possibly also the last port of call). However, there are some facts related to the history behind the information on a DM that need to be considered.
Definitive Maps were created as part of a system establishing public rights of way, set up under the National Parks and Access to the Countryside Act 1949 and developed through subsequent legislation. These maps are statutory documents which are intended to show the classification and alignment of the complete consolidated network of public rights of way in each local authority area at a given Relevant Date. Preparation of a written Definitive Statement to accompany the map was also a requirement under the legislation, providing descriptions of every right of way shown on the map.
Local authorities were required, under the 1949 Act, to prepare, review, fully update and publish Definitive Maps at regular intervals (intended to be no longer than five years). Preparation was supposed to have involved considerable research effort and consultation with landowners, district, borough, town and parish councils, user groups and the general public.
In many local authorities, this simply didn't happen to the required extent and the government eventually set a deadline (late 1950s) for full compliance with the Act. Some LAs, when faced with a monumental task in a short space of time, delegated the task of surveying to the town and parish councils. Some of these councils did a great job, some did it reasonably well and some did a poor job or just didn't do it at all.
This has led to many anomalies now seen on OS maps as, since 1960, these have been based on the DMs. Eg. RoWs changing status at county, town or parish boundaries, from a FP to a BW or vice versa, or RoW disappearing altogether at a boundary as the neighbouring council didn't complete their survey in time.
As an example, I have a RoW running along a green lane past my house. Wide enough for a car, it was clearly once an unsurfaced track used by horse and cart. It is shown as a public FP on the DM. On a site visit, the LAs RoW officer opinioned that it had clearly been wrongly classified by the parish council in the 1950s and should have been classed as a BW.
Half a mile from my house, the FP ends abruptly at the parish boundary, even though it's continuation can be clearly seen on the ground. The only conclusion, agreed with the Row officer is that it wasn't surveyed by the neighbouring parish and consequently isn't shown on the (so called) DM.