Interestingly, my TT booking has "This booking is not amendable online." when I review it.

I've not seen that before on my previous bookings & my Classic TT/MGP booking is amendable.

I booked, as usual, in the Douglas office (5 hours in the queue from 06:00!) and in the past could always amend my booking online.

I'll contact the Steam Packet to see what they say....