Before I begin, that summary does require one qualification: the genre of Royalty-dramatisations has a shockingly low average. Most of them are dreck, and a handful struggle to get to average.
The most interesting thing about The Women of Windsor is its emphasis on Fergie (Sallyanne Law) aka the Duchess of York. It doesn't play well with the title, and suggests that it was originally written and directed as a film focusing on Fergie, but re-titled to include Diana.
As a rule of thumb, films about the British Royal Family can usually be measured by the calibre of their Diana. Diana: Her True Story, for example, had a strong script and cast, but an awful Diana.
Here, the reverse is true. While much of the structure is weak - the script is soft, and some of the performances awful - Nicola Formby's Diana is spot on. She has Diana's mannerisms nailed, and delivers the performance with a rhythm that echoes Diana without actually imitating her. (A much smarter way to play real-life characters.)