It's Wellington Arch w00hoo - don't go to the wrong place!
What's in the rucksack that's so essential? Better to travel light; it's not going to be cold.
I'm likely to be there early anyway so I'm sure I'll find people and the emergency number is programmed in my phone just in case.
The essential stuff almost definitely isn't :-)
I commute carrying - tyre levers, puncture kit, spare tube, multi-tool with chain breaker, spare quick links, locks good enough for watching the bike from a distance, rubber gloves, not so mini-pump, lightweight waterproof, phone, wallet. I've added a leatherman (because I don't think the quick links can actually be fitted without it), a better screw driver (in case the lights need finagling), a space blanket (light and if I am stuck in the cold and wet it'll be useful). Aside from that I've got some gels etc. beyond the couple I'd normally have and a couple of energy bars, the sleeves, a pair of sealskins to change in to if it gets proper wet, and a long sleeve jersey. It has a 2l platypus in it and I'll have some other food with me I reckon. Ah, and a set of knogs as the back up back up lights.
Partly this is because if I'm good for it I'll be doing a few miles after the ride (plus the ones to it) solo and have a thing about being as self sufficient as possible. I consider it a victory that I didn't put the folding tyre in to be honest...