Rod G...I have also used this ferry, but only for returning from Spain and have had no problems travelling in May/June.
However, on both occasions, when we were due to go out to Bilbao from the UK in January/February our ferry was cancelled by P&O due to:
A. The ship not being servicable in 2006
B. bad weather in the Bay of Biscay in 2007.
This year we were only given 12 hours notice of cancellation by our booking agent and we had to make hurried arrangements to travel via Calais and to find overnight campsites for our trip through France. Due to the cancellation P&O could not fit us on another sailing from Portsmouth for nearly 2 weeks?
The Bay of Biscay can be very difficult and we have spoken to people who waited in the bay for 20 hours before the weather turned and it was safe to enter Bilbao harbour and often it is these delays that have a knock on effect to the timetable so P&O just cancel a full return sailing.
Personally, we prefer the ferry to Bilbao as we only have one days drive to the campsite we go to but due to unreliability of this ferry during the winter months we are not so sure and have yet to decided what we will do next year.