As AW says it's the 29 and they run every half hour. The train station and bus station are very close to each other at Uckfield.
Here's the timetable link
Brighton Hove Bus and Coach Company
Left Lewes at 8am, arrived At Haywards Heath and waited in Queue for bus to Three Bridges. Gave up after half hour and returned to Lewes. Station staff at HH says its serious damage to the tunnel at Balcombe and they think that there will be serious delays to and from London to The South Coast...