Bikes been running really doggy lately, hardly idling unless completely warmed (as in 20m ride) and rough as hell until jet needle/main jet take over so assumed pilot was clogged etc. Dismantled today and suspicions were confirmed. ALOT of silty rusty crud in the bowls that was very loose so imagine it easily got swished around in the fuel stream / mist. Pilots were blocked with it !

Cleaned everything up really well, reset float and needles etc (uk model has adjustable needles)
Put back together, balanced the carbs and started up nice and ran a treat - better than ever in fact - really pulls now.

On to my second problem, fuel running out of airbox vent and remote tap turning fuel supply off if you put it into anything other than ON. regardless of full tank or no.
I Noticed fuel hoses were on the main tank tap back to front - reversed them to factory and now ON, and RES work without shutting fuel off completely, haven't tried PRi but assume it now works as well. Also airbox dripping has stopped. The tap/carb routing on these bikes is a real PITA. My kwak zxr's run two lines ON and RES from tank to tap then single line to carbs - the way it should be - and it's the same year as my GS so can't understand the emissions BS.

Question. Could the hoses being wrong way round on the tank petcock cause airbox to leak fuel and no position but ON to work on the remote tap?

Short answer is no, but if you had that much crud in the carbs then there's likely a lot in the fuel petcock too, which may have caused your problem. Suggest you remove tank, remove tank petcock, remove and dismantle frame petcock, then clean everything including fuel lines. Maybe fit inline filter.