Not mine, obviously. Who do they belong to? And where do they go?

They belong to P&O, and sail from Hull to either Rotterdam or Zeebrugge. Note the difference between a ferry, a cruise ferry, and a cruise liner: ferries travel by day, cruise ferries operate at night, and liners are the expensive ships that concentrate on travelling rather than arriving.

In other words, you sleep rather than sit on the deck feeling seasick?

Exactly. Just as passenger jets offer three or four different classes for travellers, P&O's cruise ferries provide not only reclining seats (£76 return for a foot passenger in high season) and same-sex shared cabins, but - if you have up to £693 to spend - special and deluxe cabins, suites and staterooms. These involve such luxuries as double beds, sea views, fresh fruit and trouser presses.

And breakfast in bed?

No. Breakfast is extra (£6 if you pre-book) and is certainly not served in bed. Dinner is a rather hefty £12.

And what if I take my car?

That costs an extra £86 return, or £135 in high season. Weekend supplements apply, too, although you can save 50% by travelling midweek. But the real surprise is a fuel surcharge levy, which is added at the time of booking and is due to "world oil price fluctuations".

How much could I expect to pay for a family trip next month to Rotterdam with a partner, two children, a car, our own cabin and breakfast?

According to the online quote service, £433.50. P&O deserves some congratulations for putting together such a useful site . You can even read the menus and inspect the cabins - which (excuse the pun) makes it one of the best launches of the year.