Bearing in mind all the above. What about if the 3rd party supplier actually advertises their batteries as for example
For xxxx manufacturers yyyy ventilator?
My legal advice (a QC) is that in a court of law the 3rd party would be liable if you could proove you bought the batteries as per the 3rd partys statement.
Therfore if you have relevant 3rd party literature that is directly linked to the batteries fitted you are covered. As its upto the 3rd party to ensure the batteries are fit for purpose. Especially if the 3rd party is a proven NHS supplier.