OK cheers, I'll do some investigating and see what I can find. A quick look in the logs came up with some errors:
3 errors related to GameData/GPOSpeedFuelPump/Patches/FuelSwitch.cfg
1 error related to GameData/GPOSpeedFuelPump/Patches/ProceduralTanks.cfg
1 error related to GameData/GPOSpeedFuelPump/Patches/StationScience.cfg
[ERR 07:10:33.299] [ModuleManager] Error - more than one pass specifier on a node: GPOSpeedFuelPump/Patches/FuelSwitch/@PART[*]:HAS[@MODULE[InterstellarFuelSwitch],!MODULE[GPOSpeedPump]]:FOR[GPOSpeedFuelPump]:FINAL
[ERR 07:10:33.300] [ModuleManager] Error - more than one pass specifier on a node: GPOSpeedFuelPump/Patches/FuelSwitch/@PART[*]:HAS[@MODULE[FSfuelSwitch],!MODULE[GPOSpeedPump]]:FOR[GPOSpeedFuelPump]:FINAL
[ERR 07:10:33.301] [ModuleManager] Error - more than one pass specifier on a node: GPOSpeedFuelPump/Patches/FuelSwitch/@PART[*]:HAS[@MODULE[ModuleB9PartSwitch],!MODULE[GPOSpeedPump]]:FOR[GPOSpeedFuelPump]:AFTER[zzz_CryoTanks]:FINAL
[ERR 07:10:33.302] [ModuleManager] Error - more than one pass specifier on a node: GPOSpeedFuelPump/Patches/ProceduralTanks/@PART[proceduralTank*]:HAS[!MODULE[GPOSpeedPump]]:FOR[GPOSpeedFuelPump]:FINAL
[ERR 07:10:33.303] [ModuleManager] Error - more than one pass specifier on a node: GPOSpeedFuelPump/Patches/StationScience/@PART[*]:HAS[@RESOURCE[Kibbal],!MODULE[GPOSpeedPump]]:FOR[GPOSpeedFuelPump]:FINAL
I'll try removing a load of mods and re-adding them one at a time and see if I can track it down.