In reply to featuresforfeet:
In England, there is a legal right to cycle on a bridleway [1] but as I understand it no legal obligation on the landowner to maintain the route in a "cycle friendly" condition.
In England, posting misleading signs on a PROW is an offence that the local council can take enforcement action against [2].
That sign as described sounds outright misleading and therefore an offence. Beware however that PROWs shown on OS maps can sometimes be shown as the wrong classification or even shown one where there is no PROW. I would recommend checking your local authority's "definitive public rights of way map" before contacting the landowner; if you contact the council directly they will of course check this as part of their process.
[1] - https://www.legislation.gov.uk/ukpga/1968/41/crossheading/public-rights-of-...
[2] - https://www.gov.uk/guidance/public-rights-of-way-landowner-responsibilities
Post edited at 16:19