Caution: spoilers.
I think both plays are comedies, light and frothy confections designed to amuse. Unfortunately, Blake's 7 wasn't, though I love it for its wit. I'm not sure how much of the series Barry Letts actually watched, as he seems to know little about the background or the characters apart from the broadest outlines. In short, I was entertained, but disappointed that neither play can be considered part of canon. In fact, they're almost spoofs.
I laughed a lot, not always where I was meant to. For example, the opening scene of Avon's nightmare is, sorry, excruciatingly funny to me. I love Vila's 'escapus interruptus' comment though, and he had some other good lines.
Michael Keating does a very good reprisal of Vila, sounding just as he always did, and Paul Darrow is good, if very over-the-top as Avon, but then this is season 4. Jacqueline Pearce is still very much Servalan, if a little croaky. Did she have a cold? If so, it hardly mattered, as she just sounds more sexy and vampish. Peter Tuddenham also did well with Orac and Slave.
I enjoyed the guest characters. Dr Kapple, the aging nerd, is funny, and King Gheblakon, though an appalling man who can laugh at priestesses committing suicide, is oddly likeable, with his own eccentric concept of fair play. Yes, he is a larger-than-life character, but he is the absolute ruler of a planet full of people who consider torture and executions to be entertainment. I liked the vignettes, particularly the guard in the fuel store and the two Federation troopers in Servalan's quarters, and Officer Gradzil who seems incapable of anything more communicative than a grunt.
Dayna's comment about Furno is priceless and a very nice touch: "What a horrible place, nothing but rocks. It’s like an enormous quarry." Vila's reaction to the monsters is great too: "No, don’t fire at it, you’ll make it cross!" I also loved Avon's indignant "daylight robbery" comment as he prepared to do some of his own.
There were inventive touches, like the psionic defences around Servalan's quarters, and the teleport malfunction victimising poor Vila and producing two Avons. I did enjoy their mutual admiration when they realised what had happened, and Orac's comment that Vila is almost as stupid as Avon—pity Avon wasn't there to hear it. And a pity the crew never learned not to turn Orac off before getting all the relevant data, but that behaviour is canonical.
I enjoyed Vila's indignant rave about the Avons' ingratitude, very in character, both in its content and in Vila talking to himself. And he was quite right.
Most of the time Steven Pacey does not sound much like Tarrant, but there are flashes. Neither Angela Bruce nor Paula Wilcox sound at all like Dayna and Soolin—couldn't they have tried? A good actor should be able to approximate another's voice and accent.
Letts obviously knows very little of the background and characters of Blake's 7. Tarrant is a Federation officer and experienced pilot who received extensive training, even attending the Space Research Institute, and he should have known which fuel cells to get, or at least have the brains to ask Orac or Slave beforehand. He also wouldn't have had a 'flivver-copter' at 17, given that the Federation bans private transport.
In the series, Slave only called Avon 'master'—here he calls them all 'master' and 'mistress'.
Avon would never say "Well, she always seems to be dressed for a party, does our Servalan." It's more suited to Vila's Delta style of speech. Avon would also never refer to Servalan as 'my lady', or boast that he has torn out the throat of a tiger with his bare hands. Yeah, right.
I can't see Dayna, the youngest, calling herself 'mummy' to the crew, even though she's joking. And as for Vila saying to Soolin, "Hey, did you forget to have a shower this morning, or have I trodden in something?" in the smelly alley—he just wouldn't. He might flirt, but he wouldn't make that sort of crassly insulting remark.
I also thought it unlikely that Servalan would send Vila back unharmed once she had what she wanted. After all, she did tell Vledka not to kill him earlier; surely she would want to keep him for a suitably public trial and execution to advance her career.
Inconsistency with the series is bad enough, but in the same play? Orac says about the two Avons that "There is literally nothing to choose between them" but later says that the first one "will be sharper, quicker."
And what were the Avons going on about with all that stuff about no feeling or pain, and the emptiness of the desert?
Right up front: hyperspace. I have nothing against hyperspace—as a friend says, she keeps all her hypers there (thanks, Marian)—and my first impression was wrong: it does actually get a mention in Blake's 7. However they don't use Babylon 5-style hyper-jumps, and if hyperspace is what the Liberator and other ships pass through at FTL speeds, it looks much the same as ordinary space in B7, with stars always visible and neither red- or blue-shifted. If it were possible to hide in hyperspace as Vila does with Scorpio off the Devarni planet, I'm sure both the Liberator and Scorpio would have done it in the series. Instead, ships were quite able to see each other and indeed fire on each other.
Avon was, twice, quite happy to let Vila be executed. Avon might never admit any attachment to anyone else, but this was out of character. He was not willing to let Vila and Tarrant die in 'Headhunter', and even in 'Orbit', he hesitates briefly, horrified at what he is about to do, before he starts to hunt Vila. He may have sent crewmembers into danger, but he would not happily abandon one to be tortured and killed.
Vila has an eating disorder, stuffing his face at every opportunity. What the hell was that about? A cheap laugh? He even enjoys a lizard burger, and I can't see squeamish, perhaps even vegetarian, Vila doing that. If fear makes him hungry, he'd have been eating himself to a coronary in the TV series. Vila's funny enough the way he is without contriving a new dependency.