Whilst I'm not going to carry much different from my normal commuting tool kit, after we've crossed Clapham Common, the ride will start to spread out, and we'll have the mechanics and minibus at the rear, so it would seem daft for me to try and deal with any mechanicals which they'll be in a better position to deal with. I'll move up the field a bit, and try and stay somewhere more in the "middle" (wherever that'll be!), and at least have a chance of helping to deal with any problems before the mechanics et al catch up, which presumably could even be over the order of an hour or two later, if the ride really spreads out.
The weather looks like it'll be very good. Addmitedley there's a slight headwind, but the temperature is going to be mild, to say the least! I may well wear a long sleeved jersey, simply because it may be a bit brisk to wear a short sleeved jersey when standing around waiting for stuff, but I doubt I'll need more than that one layer! (... or gloves, or overshoes, or waterproofs)
It should be a fun evening.