The Seeds of Doom was the last six-part story featuring Sarah Jane Smith, a rather good romp featuring two episodes set in Antarctica (a venue Sarah subsequently returned to in one of the audio plays), a very camp megalomaniac millionaire, and of course the giant plant monster, the Krynoid. The background music is good too, jarring and discordant. The oddest thing about the story is that it shows the Doctor as very much part of the British establishment, in a way almost remiscent of The War Machines rather than of the Third Doctor's era. His comfort with the existing social order here is a startling contrast to what I generally consider as the Fourth Doctor's instinctive liberal anarchism, culminating in wiping out the monster by the conventional fire-power of the RAF. The establishment figures themselves are nostalgic stereotypes of an earlier era, but it's very much redeemed by the baddies, who actually undergo character development rare in any Who story.
I remembered The Sunmakers from its first broadcast in 1977, but had forgotten quite how good it is. In total contrast to The Seeds of Doom, here we have the Doctor fomenting a popular uprising against an oppressive regime. There are numerous classic sf tropes - the rag-tag rebels living in the bowels of the city, the drugs in the air supply - but also a couple of Robert Holmes touches, such as the repeated digs at the British tax system. The bad guys - Gatherer Hade and the Collector - are gratifyingly over the top, but at the same time the implied violence is pretty alarming - the Doctor almost gets his brains burnt out, Leela is almost executed by public steaming, both are threatened with ugly death by the suspicious rebels, and these seem like serious threats. Indeed I seem to remember reading somewhere that at one point there was a plan for Leela to be killed off in this story, which would certainly have been a more in-character departure than what actually happened (but would have deprived us of her in the much later Gallifrey audios). It is also, and this I think is very unusual, a good story for K9: he starts and ends by beating the Doctor at chess, and takes the initiative at several crucial points during proceedings. It seems almost churlish after all that to point out that the actual setting - humanity has been forcibly displaced to Pluto as a result of fiendish capitalist exploitation - is pretty implausible even for Who, and does great violence to any attempts to construct a future history of the Whoniverse.
I remember Underworld from its first broadcast as nothing special, an impression confirmed on re-watching. To get straight to the point: the awful, awful use of CSO to show our heroes wandering through the caverns kills the story stone dead in episode 2 (and is then surpassed in awfulness by the Doctor, Leela and Idas floating down the zero-gravity shaft in episode 3). In fact, apart from The Space Museum, I can't think of another story whose promising first episode is let down so badly by what follows. My classics-minded friends will find some meat to pick off the bones of the references to the Argonauts, but even this is rather clumsily handled - one can almost imagine the character brief: "My name is Herick! I am the equivalent of Heracles/Hercules! I like to shout a lot and kill people!" The Time Lords are given yet more unconvincing back-story, this time as penitently unsuccessful liberal imperialists. The one bright spark in a dim and unconvincingly lit landscape is Louise Jameson as Leela, who gets a good dramatic range and even develops interesting chemistry with Idas - rather more so than with Andred.
Ah, the late 70s were different days! Back then, Mrs T had only just come to power, and the unions still revelled in their ability to topple governments and, er, stop Doctor Who stories from being completed. If Shada had been finished - the last six-part story ever attempted - I fear it would be counted more towards the Invasion of Time than the Genesis of the Daleks end of the spectrum. There are some good bits - Douglas Adams' script is witty; the invisible spaceship is well done (unlike the mind-absorbing floaty ball thing); for a Cambridge graduate like myself, the scenes of Tom Baker cycling through the streets are terrific nostalgia - but the two key characters, Professor Chronotis and Skagra, seem horrendously miscast and unconvincing, and nobody quite seems to know what they are doing, which is not surprising as the story itself makes very little sense. Also I didn't like the undergraduate humour of the college porter calling the police at the very end. The Big Finish remake with Paul McGann, especially the webcast version available on the BBC website, is far superior.
In summary: first two great, second two less so.