P 

You can find CIS46 at CIS46. Revision 2 - Provision of welfare facilities at transient construction sites (sjstoilethire.co.uk)

However, it has long since been superseded.

Your starting point would be Provision of welfare facilities during construction work (hse.gov.uk)

The only obvious legislation is that within the Construction (Design and Management) Regulations 2015, assuming the sites are in Great Britain, or the parallel provisions in the Workplace (HSW) Regs 1992 [or other sector specific Regs]

As for case law, depends what you mean by "case law".

If you actually mean authoritative judgments from the Courts then you are very unlikely to find any. It would require a prosecution or civil case (civil case alleging perhaps occupational ill health caused by breach of statutory duty) that has been defended up to a sufficient high enough Court to set legal precedent. 

You could look for cases that have been taken to Court but it may be difficult to separate out those on transient sites (construction or otherwise).

If you find press releases then they may be HSE stating that a case has been completed with a conviction (USUALLY with the defendant opting NOT to go to trial - in which case no precedent whatsoever) and the HSE may or may not have commented what it thought might have been reasonably practicable -  HSE press releases are often silent when it comes to suggesting what WOULD have been reasonably practicable [whatever the topic!]

You could try looking up prosecutions on the HSE database - www.hse.gov.uk/prosecutions and then search both the current and historic [up to 10 years] by BREACH - so with an ADVANCED SEARCH you could narrow down to the right bit(s) of CDM or Workplace Regs.

But for some perspective on the current database [convictions in the last 12 months] there are only 25 cases involving any Regulation within CDM. 

Suppose we guestimate that there might be about 300 CDM cases in the historic database [there WILL be a higher number per year going back] then may be of those 300 a maximum of perhaps 30 would deal specifically with welfare issues and of those 30 only a fraction would relate to transient sites.

So you might be looking for a needle in a haystack!

In terms of "bolster"ing the case - you need to consider the occupational health risks on top of the basic welfare requirements.

P