You know what I'm going to say! The PC said don't do it, but you went ahead anyway...
On the more general point, all expenses are reimbursed retrospectively, but all expenditure should be approved before being incurred. Whether or not the budget holder has discretion would depend upon the wording of your Finance Regs and scheme of delegation for your committees. All options are possible. I presume that you have the minutes of the meetings at which your expenses were discussed/agreed/rejected? It isn't clear from your wording whether you are talking about individuals or committees, but obviously only the latter can have powers to spend.
Check the paper trail and see what you find.