So about 3 months after I had my solar panels installed, Octopus finally set up an export tariff so they could pay me for any excess I send back to the grid.
Nicely timed so that we've missed the time of year when I was actually sending a lot of excess electricity back to the grid.
I've been in touch to see if they'll credit me for the electricity I exported before the export tariff was set up, and they've said no.
Formal complaint made this morning. Let's see what happens.