London Road Caliper - Head scratch

ed1973 Posts: 284
Hi all
I’ve got a Planet X London Road project I’m on at the mo. Got sent a nice groupset with some trp mechanical brakes. Front brake caliper was no problem, rear caliper would not go on. LBS said I need an Avid BB5 or BB7 as it’s a post mount on the rear. I’m fine getting a BB5 from wiggle (other shops are available but wiggle seem the cheapest). Just looking at how to attach it and just can’t seem to picture it.
Are the bolts provided long enough?
PS Planet X were also helpful and said BB5 would be fine