Mike P, To have had only £209 returned and to be charged £181 for 17 days cover seems unreasonable to say the least. 17 days pro-rata charge on your annual premium of £390 would be £18.16. As Flux have retained £181 of your premium, this means you've been charged £162.84 'admin' charges for processing the cancellation of your policy, which will involve someone processing a simple transaction at a computer terminal that should take no more than a few minutes.
Have you checked your insurance policy to see what it says in relation to cancellation / admin charges, or did Flux provide you with any documentation when you bought your policy that outlined their charges? If not, then I'd ask Flux to explain their admin charges and to justify what you've been charged.
In my motor insurance policy, it states that an admin fee of 'up to' £38 will be charged if I cancel my policy during the period of insurance. If this charge is representative of the motor insurance market in general, then what you've been charged looks totally unreasonable.
If you do decide to ask for an explanation and you're not happy with the response, you could complain to Flux's CEO - details at the link below. Good luck, and please let us know the outcome.
http://www.ceoemail.com/s.php?id=ceo-10469