In reply to Co1in H:
How close are other houses in your road to the river? if they are right on the stream it could be effecting the whole postcode
PM me your postcode and house number and I'll look it up on the flood database and tell you what risks are flagged.
What you'll find is some insurers are very sophisticated with flooding and others aren't, it sounds like you are talking to insurers that rate based on the entire postcode or perhaps even just a portion of it.... ie. GU33 6 (instead of GU33 6LJ). Insurers with more sophistication will look at the exact location.
Another possibility is there is a flood risk you aren't aware of up stream from you... e.g. in the next road there could be a restriction that when the stream is in spate means it could divert.
These insurers that you have contacted - they should point you towards an insurer that will quote (its part of the insurance agreement with the government that no house is "uninsurable"- that's not to say it might be pricey)
Post edited at 21:32