Please do clarify your requirements and expectations.
Could you clarify the volume of work you're expecting for the 5hrs/week stated please? Your budget (max. £60 a week) is very low for an experienced writer but depends on what you're expecting for that fee.
Keep in mind that you have more chances to win a Job with Case Study as fewer Sellers will send a proposal. Moreover the Buyer has already paid a fee for it, which shows that they seriously intend to award the Job.