Hi Mark

I share your concern not least given the guidance at Your garden walls: better to be safe - GOV.UK

Considerations for both the Council and the owners both under occupational health and safety legislation (principally Sections 3 and 4 of HSWA but also for the owner/whoever has let out a Contract to you under e.g. CDM) and also under the Occupier's Liability Acts.

In simple terms the latter require the Occupier to do what is "reasonable" to keep those liable to be harmed safe, and what that entails is dependent on the level of risk.

One of the things to take into consideration is whether putting up fencing etc will obstruct the public highway which brings in lots of other rules to contend with.

You might have a clear steer from the "landlord" but are they the owner?

Does the owner even know that you are having this debate with the landlord (assuming not one and the same)?

Do you have an engineering assessment of this wall or is it a case that it looks a bit dodgy, which might be the case but, particularly if a historic structure, then not necessarily something to raise a red flag about? One of the problems is that most structural engineers and building surveyors have been educated to assume that everything should conform to Eurocodes and sometimes may come up with OTT solutions to deal with old structures that have stood the test of time.

I think you need to escalate this to the owner (if they are not the landlord) and give them the chance to take ownership of the risk, possibly even taking on CDM Designer duties. 

What is "reasonable" or "reasonably practicable" for the owner might well be more onerous than what is "reasonably practicable" for you as a Contractor to effect. Lots of things come down to who has what extent of control over the state of the premises.

Good luck, Peter