Hi Andy - it looks like mildew to me. A bit of a surprise to find it on the dodgers/backrests as it's more common down below when things get damp and airless. I have a couple of suggestions, ACR Boracol 5RH is mostly used to treat teak, but a knowledgeable lady who was complimenting me on my nice clean silvery rubbing strakes told me it was equally effective on canvas. I use it once or twice a year on my teak and canvas and it seems to be pretty effective. It's certainly very effective for keeping the green algal mould away, how effective it will be on mildew I'm not sure. It's very easy to use, you just paint it on with a brush and leave it. Nothing happens immediately, but after a couple of weeks you should see that all traces of mould have disappeared, and it stays that way for several months at least. It also doesn't seem to bleach the material at all, and I have tan canvas which would show bleaching fairly readily I think. The downside of Boracol is that it's only sold in 5l containers at around £35+, and it's also been reclassified for 'professional use only' so some places won't sell it to Joe Public.
The other recommendation is Wet & Forget. This was recommended by a professional yacht maintenance guy I met while installing my new engine earlier this year. It's mainly aimed at patio and drive cleaning, but also works on boats. He said he uses it a lot and it's very effective. Also very easy to use like Boracol, just brush it on and leave it.
My wife suggested Vanish Oxi-action, which she has used on mildewed fabrics with some success, but mostly on things that can go in the washing machine. Had a buoyancy aid with some mildew and it improved it but didn't completely remove the marks.
Last suggestion (probably not helpful) time for new canvas??
Good luck
Nick
