Called the silver line and got through after about 30 mins. We were originally booked on the first flight AM on 27th to Hannover (then getting a train to Berlin for the marathon weekend, because direct flight to Berlin was crazy expensive, as expected). Have now been re-booked on the 7.20pm on 26th but now direct to Berlin, which is perfect as no extra time off work needed and the extra night's hotel will be outweighed by not having to purchase two train tickets from Hannover-Berlin. Also 20 TPs instead of 10 and more avios due to further distance. The only additional cost was adding an extra day parking at T5 for about £20. All in all, it's worked out great for us and the agent was helpful and it was all sorted within about 5 minutes of getting through. Furthermore, managed to bag the last couple of exit row seats. Appreciate it may not work out this well for many others!