begin{filecontents}{jobname.bib}
@jurisdiction{case,
  title             = {{Somebody} v {Somebody Else} and Others (No 4)},
  note              = {[2016] AC 100},
  pages             = {100--178},
  date              = {2015-03-10,2015-03-12,2015-04-25,2015-04-27,2015-05-01}}


@legislation{deddf,
    title               =   {{Deddf  Ardderchog (Cymru)} 2014},
    year                =   2014,
    author              =   {{Cynulliad Cenedlaethol Cymru}},
    languageid          =   {cy}}

@article{article,
  title     = {Fantastic Findings and Where to Find Them},
  author    = {Vark, Aar D.},
  journal   = {Diligent Discoveries},
  volume    = 45,
  number    = 3,
  pages     = {23--32},
  year      = 2010}

@book{book,
  title     = {Brilliant Baths},
  author    = {Allard, M.},
  publisher = {Empire Books},
  address   = {Alpha Centauri},
  year      = 1998}

end{filecontents}

documentclass[welsh,british,a4paper]{article}
usepackage[utf8]{inputenc}
usepackage{geometry}
usepackage{cfr-lm}
usepackage{babel}
usepackage{csquotes}
usepackage[style=authoryear-comp,backend=biber]{biblatex}
bibliography{jobname}

begin{document}

Most of my bibliography entries are `normal' sources, such as articles autocite[e.g.][]{article} and books autocite[e.g.][]{book}.
Textcite{article} and textcite{book} are cited using the author-year system.

However, I also need to cite legal sources, even though I'm not working in law or writing in this discipline.
Nonetheless, these need to be cited appropriately.
Unfortunately, Biblatex doesn't handle these out-of-the-box and the contributed styles which do are all designed for work in law, as opposed to, say, history, philosophy or quantum mechanics.
Hence, the standard styles do not handle these cases well:
begin{quote}
  Textcite{deddf} is a law autocite{deddf}.
  Textcite{case} is a case autocite{case}.
end{quote}
I emph{think} that these should be cited rather differently, although I'm not certain exactly how they should look.
I suspect something like the following is needed
begin{quote}
  Deddf  Ardderchog (Cymru) 2014 is a law (Deddf  Ardderchog (Cymru) 2014).
  emph{{Somebody} v {Somebody Else} and Others (No 4)} [2016] AC 100 is a case (emph{{Somebody} v {Somebody Else} and Others (No 4)} [2016] AC 100).
end{quote}
That is, specifically legal sources should follow the Oxford Standard for the Citation of Legal authorities (OSCOLA), except that inline rather than footnote citations are used.

Similar principles will also apply to the entries in the bibliography.

printbibliography

The entries for the book and the article are as desired.
The entries for the law and the case are not.

The year of legislation is part of the name of the act and shouldn't be separated from it.
The title should come first.
Probably I should not have an author but just a publisher or URL, but if I don't have an author, author-year citations go wrong.

I should not have `n.d.' in the entry for the case, but nor should I have the year in round brackets.
No full-stop should follow the year.
Not sure about the full-stops at the end of entries, but OSCLA doesn't use them, I don't think.

In some cases, citations and entries for cases are more complex, though I haven't yet got to grips with all the nuances.

What is the best way to configure Biblatex for legal sources, when using a non-legal style for all others?

end{document}