So I met today with the (very apologetic) sales manager at the dealership, everyone there seems to be quite embarrassed and quite appreciative I've been so calm about it all. Surprised even myself I must say!

They're sorting out some FoC extras for me and three years free servicing, to make up for the hassle of having to wait for a new car to be ordered and having to drive a diesel Jetta (S level trim) until my GTI arrives. Overall I'm pretty happy, as I'd already managed to secure over five percent off list so I feel like for a dealership I'm not doing too badly price-wise, and frankly being patient and sticking with them gives me a stronger position for leverage longer term!
They're going to explore stealing a GTI that's already in the system but not been confirmed yet, but the chances are apparently quite slim. Sadly it means I'm going to be joining BW23/24

At least as the sales manager put it, I've been through this process before so I know how the timings work, etc. - I think he was pleased to have the chance to put it right rather than having to explain to his managers why he'd had to give a customer their deposit back!