Salvia splendens or Pelargonium (what people call Geranium). These are readily available at Garden centres, or available as seed or plug plants. Or red Bizzie Lizzies or Begonias if the site is shady. You would need one every 6inches or so.

go for a plant that stands any weather the last few years buzy lizzies have struggled in the wet,salvia or pelargonium would be my choice--good luck---

The great thing about the pelagoniums is that they are really tough as long as they dont get frosted. They are also easy to take cuttings from if you have a window ledge you can overwinter them on.

How about a begonia - follow this link for an example: http://www.spaldingbulb.co.uk/r_13809-042/Begonias/Giant-Patio-Begonia-Red-Corm.htm