But does anybody know which is the outgoing and which is the return fuel line from the fuel filter? I'm trying to run some diesel purge and having no luck. The only thing I could find online was the attached picture. I wasn't able to get the engine to run on diesel purge and it just wasn't having it, and I just want to rule out the possibility that I had the lines hooked up backwards. Is the attached picture correct?

https://workshop-manuals.com/skoda/octavia-mk2/power_unit/1.6/55;_66;_77_kw_tdi_cr_engine/fuel_supply_gas_operation/removing_and_installing_parts_of_the_fuel_supply_system/fuel_filter_summary_of_components/

You're a lifesaver thank you! I must have looked a right idiot funnel-priming a return line while the supply line had a backwards filter installed on it and was bone dry 😐