hi Erik, firstly apologies, we've been on travels followed by a training course, so I have only just got back to read your message.
The coaches run just about every hour from Southampton Central. They are usually 2/3rds full, so you can buy a ticket from the office next to the pick up point (below the Asda shop in West QuayRetail Park - 5 minutes taxi ride from the terminal) but these can be more expensive than buying online at http://www.nationalexpress.com/home/hp.cfm
your insurance should cover this if the ship is late & you have to buy replacement tickets. the Nat Express booking process will also assume you want their £1 insurance unless you deselect it.
Hope this helps with the planning, feel free to add more questions, or PM us if needed,
have a great weekend,
Alan & Katrina
Last: P&O Oceana December 2016
Next: NCL Jade August 2017
Then: P&O Ventura December 2017
Golden, Grand(2), Caribbean, Royal, Sea(4+1 aborted)
Adonia(4+2), Arcadia, Artemis(2), Aurora(5), Azura(2), Britannia, Oceana(4), Oriana(12), Ventura(2), Victoria(5)
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Oceania Nautica, Minerva, Minerva II, Fred.Olsen(8), NCL Jade, =X= Eclipse, HAL Ryndam, Aza.Journey, CMV Magellan