firstly you are getting ripped off paying that amount!
Secondly unless your insurance company is really anal..they they wont cancel your policy straight away. Ive missed payments two or three times, and I got a letter giving me 10days to pay or else the policy would be cancelled. Basically the reason behind it was because I had changed jobs and instead of getting paid on the same date each month, im now paid four weekly.. If you have the money, ring them and pay it in the morning, if you dont, ring them anyway and tell them when you will be able to pay it.. and make sure you keep to that promise.. Any decent insurer will understand that occasionally people do have a bit of financal difficulty so should be willing to help.
Though that said.... it prob wouldnt be a big problem if they did cancel it as then you could get covered somewhere cheaper! Like said above... try Quinn Direct!