170mm cranks ok at HH?

I Already Own a Fixed Wheel Bike – Can I Ride That?

In most cases this is fine, Herne Hill has shallow banking compared to indoor tracks so crank length / bottom bracket height is not a big issue.
Bring rides a pista with 170s on I'm certain.

put it this way, Manchester spec 165 on all track bikes, they say 170s only on bikes with "manchester spec" ie; an even higher BB.

and you use eggbeater pedals, which have a low profile and won't clip as soon as flatties with clips n' straps will.

to summarise: you'll be fine.. the instance when you may come close to a strike can only be cutting sharply up the steepest part of the banking at a low speed which isn't gonna happen unless you are on mushrooms