I am travelling to the USA from england next month, after about 60 days travelling i am going to cuba for about a week, and then I'm hoping to go to cancun and from there to Hawaii, would i be able to spend another full 90 days in hawaii before having to leave, as I've previously left the US, or would it just be 30 days, to make up 90 from the time i already spent in the states? any advice would be very much appreciated!
There is no hard and fast rule, but in general you'll want to spend more time outside the US than inside.
Cuba and Mexico do not restore the clock, as stated by Timatic, and the clock will continue while you're in these countries.
So if you spend 60 days in the US, then 7 days in Cuba, then (for example) 3 days in Cancun, you will have 90-60-7-3=20 days left in the US