Apologies if already discussed. The NHS website says you can book an appointment for a booster dose if you're over 40 and are 152 days/5 months out from your second dose. The appointment date will be from 182 days out from your second dose, so you'll still have to wait a month - usual at-risk rules apply.
I'm currently at Day 150, and given the way Christmas and Boxing Day fall this year, I'm guessing my earliest date for a booster would be December 29th. (Can't see me getting it done Christmas Eve.)