May be a bit late, but VAT refund in T5 for checked baggage is VERY easy. I did it last week and was very surprised when HMRC landside turned me away and told me to go directly to the Bureau de change around the corner. They did the inspection there,stamped the form, and processed the refund on the spot. So it doesn't really matter if HMRC is open.
This is the only airport/terminal where I have seen the bureau de change do the inspection and stamp the form.