Where a fire assembly point for a hall is accessed by a private roadway owned by a third party (over which there is a right of way for hall users), would the hall management be liable for any slips/trips that occurred on the roadway as a result of poor maintenance of its surface?

The landowner of the roadway would have primary responsibility under the Occupiers Liability Act - whether the hall management could share culpability would be one for the lawyers, however the landowner would be the expected target of any litigation if there is any risk of users suffering injury on the premises by reason of any danger due to the state of the premises or to things done or omitted to be done on them.