Estates organise the PAT testing, I assume they do get an inventory but the amount of happy smiling face stickers we see is massive, it's sort of like the PAT testing equivalent of the elves of the tailor of Gloucester (suddenly appears in the morning) For all I know they could just be sticking the stickers on (just hypothetically speaking here also). Actually in some cases I hope they are since I've seen them on Bipap's, diathermy machines (how the hell did they get into theatres!) and one piece of equipment that the staff know doesn't like PAT tests. (and they weren't happy to see the sticker either)
I think basically you get what you pay for and perhaps the answer is in-house testing or get someone to watch over their shoulder (but then again that might defeat the purpose)
