I searched online and found some advice, which I followed, and it worked

First I used sellotape to lift off all the pollen. It took quite a while to really get off all the loose pollen, leaving just a yellow stain. On the web, the advice I saw was to then hang the item in the sun to bleach the stain away. However, this is England, so I decided not to wait for the sun!

Instead, I used a bar of stain removing soap (here in the UK, it's called Vanish), lathering it up and rubbing on the stain, leaving for a while, then rinsing and repeating. I did this three or four times until the stain had noticeably faded quite a bit, then washed the coat in the washing machine as per the label. The stain has completely gone