Thanks everyone, I found a creative way (or thought I had) to change the seat, I started booking one ticket and selected the middle seat in row 1, which was the only seat available. I then started a different booking and all of a sudden, row 2 had come into the plus section, so I selected 2A.
However, after booking I went in to have a look, and they had put me back in 1B! By that time, however, 1F had opened up for some reason, so all ended ok in the end.
I get that they are two difference classes of service, but to book the most expensive ticket and end up with a middle seat being the only choice, seems unfair. I checked EF and seats are wide open for the Go section.