How to specify default printer tray?

I'm using CUPS "lp" on Debian Sarge to print to a HP Business Inkjet 1200 with two paper trays. I want to use the lower paper tray, but it seems no matter how I edit the PPD, it always uses the upper tray. I'm using the hpijs driver, as recommended. The relevant material from the PPD is posted below.

Can anybody please tell me how I can make the lower tray be used?

Thanks.

Tony.

PPD file extract:
-------------------

*OpenUI *InputSlot/Media Source: PickOne
*FoomaticRIPOption InputSlot: enum CmdLine C
*OrderDependency: 100 AnySetup *InputSlot
*DefaultInputSlot: Lower
*InputSlot Lower/Lower Tray: "%% FoomaticRIPOptionSetting: InputSlot=Lower"
*FoomaticRIPOptionSetting InputSlot=Lower: ",PS:MediaPosition=4"
*InputSlot Default/Printer default: "%% FoomaticRIPOptionSetting: InputSlot=Default"
*FoomaticRIPOptionSetting InputSlot=Default: ",PS:MediaPosition=7"
*InputSlot PhotoTray/Photo Tray: "%% FoomaticRIPOptionSetting: InputSlot=PhotoTray"
*FoomaticRIPOptionSetting InputSlot=PhotoTray: ",PS:MediaPosition=6"
*InputSlot Upper/Upper Tray: "%% FoomaticRIPOptionSetting: InputSlot=Upper"
*FoomaticRIPOptionSetting InputSlot=Upper: ",PS:MediaPosition=1"
*InputSlot Envelope/Envelope Feeder: "%% FoomaticRIPOptionSetting: InputSlot=Envelope"
*FoomaticRIPOptionSetting InputSlot=Envelope: ",PS:MediaPosition=3"
*InputSlot LargeCapacity/Large Capacity Tray: "%% FoomaticRIPOptionSetting: InputSlot=LargeCapacity"
*FoomaticRIPOptionSetting InputSlot=LargeCapacity: ",PS:MediaPosition=5"
*End
*InputSlot Manual/Manual Feeder: "%% FoomaticRIPOptionSetting: InputSlot=Manual"
*FoomaticRIPOptionSetting InputSlot=Manual: ",PS:MediaPosition=2"
*InputSlot MPTray/Multi Purpose Tray: "%% FoomaticRIPOptionSetting: InputSlot=MPTray"
*FoomaticRIPOptionSetting InputSlot=MPTray: ",PS:MediaPosition=8"
*CloseUI: *InputSlot


(I posted this query to the linuxprinting.org forum a few days ago, but there was no reply)