A better interpretation for you, and entirely appropriate in my opinion is that the rolling budget means that you have a rolling purchase order of £200. You invoice as normal on delivery, and expect payment 60 days later, but having invoiced for some work, the purchase order commitment increases by the sum you have just invoice, to another £200 of commitment on the slate, unless they guillotine it and inform you otherwise.
It sounds like a way of their finance/ credit control department keeping tabs on local commitment made by shareholders without having to confirm a new purchase order each time.
Check with them to see if they are agreeable to that.