Right

My insurance policy finishes in September on the GT

It will be cheap enough then for me to get insured on a G40

Trouble is i don't think i will have enough saved up to buy the G40 in September and i know i don't want to take out another years insurance on the GT

What should i do? Is there a way around this

Seems stupid that you can only buy a car when your insurance runs out or am i missing something here?

Take out a year's insurance on your GT as that will be the car you'll most likely have at the time.

Then, when you have the money for a G40, just contact your insurance company and tell them you've changed cars. They should adjust the policy details and reissue a certificate for your new G40.

you can change cars whenever you want. just phone the insurance company and tell them you've got a new car, they'll change your policy details and charge you/alter your monthly payments for the new car for the rest of the policy period.