Is it true that a public right of way ceases to exist if it is not used for 20 years?
No – this is a fallacy, except in Scotland. Elsewhere in the UK public rights of way continue to exist indefinitely unless the land they cross is destroyed (for example by coastal erosion), or extinguished by official order. However, the opposite is true, as a public right of way can be acquired in certain circumstances where there was none previously if a route is used for 20 years.