Recently managed to borrow this from one of the guys from my local Photographic club
http://www.manfrotto.com/product/8374.76909.77462.0.0/680B/_/Compact_4-Section_Monopod
The Manfrotto 680B. Sturdy monopod, appears to be around 40quid for the pod itself. Admittedly I didn't use a head with it as all i was doing was panning at the footy with my 70 - 200 with extender screwed directly onto the adaptor ring on the lens. Depending on what you're using it for with your 100 - 400 this may be an option unless you cant manage without a proper head on top as well.
Haha, i just put the 680B on the top of my list along with the 496RC2 head