Update:
Sent complaint letter 16:23 to the Service Director as above.
Received email at 17:03 to say he's in a meeting and will call back as soon as he's out.
Received call from Service Manager at WLMG Mill Hill 17:05 to say she has received the forward email from the Director and she wants to help. I explained the situation, she offered to personally see me at my own convenient time, which will be Tuesday 10:30am.
Received call from the Service Director at 17:43 to follow up and ask me to keep in touch for a way to compensate this and restore faith in Nissan and WLMG brands after the problem has been dealt with.
Called WLMG, a lady from Bookings for service took the call, promised that one of the managers will call me within 10 minutes. This was 12:00, it's now 13:06...