I do believe I recognise these books. I don't think the series had a name - just the individual book titles - but Goodreads and Wikipedia refer to them as the "Professor Gamma" series, so let's go with that.
The authors were famous astronomer Sir Fred Hoyle and his son, Geoffrey Hoyle. And the books were indeed "Ladybird-type format", since they were published by Ladybird.
In one book, the Sun vanishes. It turns out it's been taken to use as one of a number of floodlights for a bunch of gigantic astronomical-sized space aliens to play their gigantic astronomical-sized space football under.
This sounds like The Giants of Universal Park. An Amazon reviewer describes it thus:
I think this book is perhaps the most odd out of the series, with the giants playing football inside a wacky world. It's about a villain called Lighto, who is trying to steal the sun! Once again it's an adventure for Professor Gamma, William and Kiryl. They journey to the strange world of the Universal Park ... witness a huge game of football, and Gamma must score to get the sun back
The second book would probably be The Frozen Planet of Azuron. Once again, I quote an Amazon review:
William, Professor Gamma and his daughter Kiryl set out along the energy pathways of the Universe to catch Absolute Zero, the fiend who has found a way to convert heat into action. Zero has been sucking up heat everywhere he goes in the Universe and using it for no good purpose.
Incidentally, the review also states:
The story is full of strange creatures and strange phenomena: a transparent dog which looks like it is made of crystal; ghost like entities that appear out of nowhere and an entire universe in a carrier bag.
The other two books are:
"The Energy Pirate", in which one of Professor Gamma's old friends has turned villain and is trying to steal the Earth's sugar.
"The Planet of Death", involving a villain called Viro and a virus (possibly artificially created). I think Earth was the planet targeted by the evil Viro in this book.
All four of the books in this series were published in 1982, so by 1985 they would not have been as new as you thought - maybe your local library bought the books several years after they were released? Or perhaps they bought a later printing or edition - a Google image search will reveal two different versions of "The Energy Pirate" with different cover art.
You describe them as "probably a Dr. Who knock-off". I've found a Twitter thread by someone who's read The Energy Pirate and agrees with you! He describes the book as:
feeling like a #DoctorWho s17 reject
and notes one scene in particular, where:
A boy tells his best friend Kirryl about the theft, and she resolves to tell... Her grandfather, who lives in a strange blue house with an indeterminate front door.
(link to Tweet)
"It's a barmy book" he concludes, "but I bloody love it."
EDIT: Comments on this blog post suggest that audiobooks of the stories were also released, in the form of cassette tapes.
