Hi Graham, welcome to Tom's forum.
Yes I agree, Seeds of Doom is one of the best stories of the Baker/Hinchcliffe/Holmes era; it's certainly the darkest. Hinchcliffe and Holmes were really pushing the boundaries of acceptability and releasing the handbrake on horror and violence in this story. There are some extremely powerful and quite shocking scenes of bodily mutilation, as well as Harrison Chase's grisly demise in the composter leaving little to the imagination.
There's also plenty of action, most notably the Doctor incapacitating Chase's chaffeur with a single punch, before jumping through a glass skylight and laying Scorby out with a chair! Uncharacteristic of the Doctor's usually subtle methods of dealing with his opponents, but magnificent all the same!
In spite of the fact that during the Arctic episodes it's blindingly obvious that the protagonists are wading around in polystyrene, The Seeds of Doom was beautifully directed by Douglas Camfield and is surely one of his most memorable efforts. All this plus terrific location filming and a top-notch cast make The Seeds of Doom a story worthy of an extra special DVD release, full of bonus extras.