I think that's just for the Mk4 single post jobbie on their site. I got a pair of longer ones from AA, but that was quite a few years ago (when the Mk1 was current) and I doubt they'd have any left but might be worth a try. I think it was about £15. All they are is 2 stainless rods with a chamfer on one end and drilled and tapped at the other. If you take one to a local engineering shop they should be able to knock you some up in no time. Use the same screws as you already have, it's just a case of making the rods as long as you need.
Edit: I'm sure Rowan could also run some off for you, they seemed pretty open to doing 'specials' when I was chatting with them at North Oxon.
ISP Airstream MK III No 15 which is a bit poorly at the moment and having a rest
AA Pro Sport + Ginb FT8 stock + Bushnell 4200 6-24x
EV2 MK1 + Leupold Comp 40x
Weihrauch HW100 KT laminate
SBFTC Competition Secretary