Male robins tend to have less bright colouring on their chests during the summer, and are therefore less noticeable. But they'll still be around. Food is also more plentiful everywhere in summer, so they're less likely to be hanging about than when you're digging in the garden in the winter months.
I seem to have robins all year round, and certainly have noticed some over the past several weeks (when the heatwave was still in full force). Sorry can't be more scientific!
Our own native robins are boosted by some migrant robins in the autumn, so there will be more of them around in the winter and therefore more likely to be seen.
It is not so much 'seeing' the robins that presages colder weather, but 'hearing' their 'winter song'. As the weather gets colder, the robins start to make a 'chip', 'chip', 'chip' call.
Thanks! My dad used to work in a cemetary and he always said how friendly the robins were. They would just sit there while he was digging etc. Everytime I see a robin, it reminds me of him and they do seem to be friendly, watching what you are doing.