Interesting. The new HSG274 Part 2 guidance does begin to address this issue of shared premises - check page 46 and paragraphs 2.147 onwards. It sounds like both parties will have duties - so clarifying exact roles and responsibilities between the two is key. I would suggest that your client has every right to ask questions of their landlord about the incoming water supply to their unit including the sprinklers...but the landlord also has every right to ask questions about what is being done with that water - particularly if there is a possibility of any Lp reservoir forming that may seed contamination back into the common centre supply. They can't do the risk assessment for your client though as they have no control over who goes into the store, the way the store uses the water etc. This is controlled by your client and so is their risk to control unless they've contractually moved it to the landlord.