Hi Richard Salt
The vent pipes go from the tank to evaporator small black steel box on top of tank then to plastic round box = Rochester valve this is should relief pressure at 2psi and is open when engine runs it works off vacuum from there it runs down back of tank comes out in front of NS rear wheel if you look all you might see is short steel pipe sticking down its high up .from there it should go to front NS wing back of headlight to carbon canister then pipe purges the canister from NS inlet manifold
On my cabriolet the lines from front of NS rear wheel were missing and no carbon canister inlet manifold blanked off with rubber
bung. I bypassed the Rochester valve in the boot blanked of vacuum pipe feeding it run line to front NS wing back of headlight installed carbon canister mine came from Rover 25 cost £8 eBay coupled purge line to inlet manifold this is open when engine runs with a 12v valve on canister the canister as a vent line to atmosphere I also have a coupe the lines from the front of rear wheel are missing and inlet manifold blanked off car vets to atmosphere under car front of rear wheel.