The other day in a public library I came across another one of Joan Aiken's Austen-related books, The Youngest Miss Ward. It's really not a sequel (more like a prequel?). It's related to Mansfield Park but the main character is not mentioned in MP at all, Aiken has invented her as the youngest sister of Mrs Norris, Mrs Price and Mrs Bertram. (This Miss Ward lived such a "disgraceful" life that Mrs Norris decided her name shall never be mentioned among the family, which is supposed to explain her complete lack from MP...?)
IMO, if you like the sequels Joan Aiken has written (I was constantly reminded of Jane Fairfax), you would find this a tolerable read. At least you shall not be bothered by inaccurate depiction of your favourite characters, because the only one of Austen's characters that features significantly is Mrs Norris (and I think she's pretty much in character). As far as historical inaccuracies go, I think it must have its fair share of them (though I'm not an expert so I can't point out much), and certainly the 21st century sensibilities come through a bit too clearly for me (such as feminist issues etc).
So if I may add some things for the record (trying not to include spoilers), just trying to make my feelings about this book clearer?
The plot seemed, at times, a bit too far-fetched for me. It had some of the same basic elements as Jane Fairfax (and Mansfield Park too, I guess): a girl is brought up in a family of relatives, she makes friends with a son of the family, she's talented and a lot of people don't realize that, she has a crush on an older man (like Aiken's Jane Fairfax and Mr. Knightley).
I had enjoyed Aiken's Jane Fairfax more than any of the other sequels I've had the chance to read, but this seemed to take the JF plot and go on more of a flight of imagination with it. As if Aiken had decided to add a bit more spice and melodrama and exotic elements and those seemed a bit foreign to me in a world that was supposed to be Jane Austen's (the time is before the action in Mansfield Park starts).