European Highlander, European Causeway, and European Endeavour to dry dock
According to P&O's latest timetable information leaflet here, European Causeway and European Highlander will dry dock from 30 Apr – 1 May and 28 – 29 May 2016. It is not yet clear whether any ship will be brought in to cover, though in previous years Norbay has made the trip North to operate the Cairnryan route in the absence of the either regular vessel.
In other P&O dry docking news, the pure car and truck carrier Neptune Aegli has arrived in Dublin. It is expected that she will enter service on P&O's Dublin to Liverpool service on Monday, releasing for European Endeavour for her dry docking. Neptune Aegli can carry some 1.800 cars and has 1500lm of freight space.
Update 5/4/16: Further inspection of this information indicates that no cover will be provided during the periods European Highlander and European Causeway are off service - hence the work being undertaken during quiet weekends in May. European Endeavour would appear to be at Cammell Laird's at Birkenhead for around 2 weeks.