I'm not certain what formatting "####0" represents, it could be padding with spaces or zeros, but basically it would be a CASE expression like so:
case inc_data.event_type
when 'c' then 'R'+left('00000'+cast(incident.incident_ref as varchar),5)
when 'i' then left('00000'+cast(incident.incident_ref as varchar),5)
else 'P'+left('00000'+cast(incident.incident_ref as varchar),5)
end
"Do not seek to follow in the footsteps of the wise. Instead, seek what they sought." - Matsuo Basho