I had a vaguely similar experience to Davey's although we live nowhere near the coast. Our drive also slopes towards the house and occasionally floods from surface water. We had a full width storm grille installed across the bottom of the drive and this water was piped through into our main 2cu.m.soakaway in the back garden. Then the problem became worse because under heavy rainfall the water backed up and came out in reverse through that grille! The problem was found to be that the soakaway filled up and then the rain built up inside the downpipes. This head was sufficient to send it back to the grille. As this only happens occasionally, I resolved it by drilling a 10mm hole near the bottom of the downpipes and pushing in short stub pipes so that the flow of normal rainfall was unaffected. Excess pressure was relieved and, so far, no more backflow.