Thanks, Ladybirder.
Sorry for the delayed response. I only found your reply while trying to trace another thread I'd posted in.
Anyway, I'm wondering whether we see them migrate here because, in a good berry year they end up with slight overpopulation and some are forced to forage further afield because all other (Scandinavian) territories are held by established pairs, or if it's because we see them however good/bad the crop is because the entire berry crop gets eaten and the whole population has to up sticks and spread all over Europe, in search of their favourite food.
If the latter case applies, the poor berry crop in the breeding area obviously means that they'll be mere handfuls of field observations as the population is scattered over a huge area.