Just because I was really bored, and sort of curious, I took a copy of both this photo and the close up into photoshop. By changing the contrast and brightness levels, it becomes clear that the only obviously missing letters on the door are in fact "& S4." I then matched the letter size and added the missing items back.

Keeping in mind the fact that Motts took this picture at an angle and not straight on, when you rotate the image accordingly there is still a minor distortion of angle, but it does seem to appear that with the accounting for that and the added lettering returned, it is all in fact centered on the door itself.

The EC list posted above shows that S1 means the items are to be kept locked up, as evidenced by the well used lock on the door. S4 states the items in question are to be kept out of living quarters.

Since this was a hospital it would only make sense that anything that could be classed as a poison would be required to be locked up, and certainly out of the range of the patients living areas.

As for the internal use part...well that could mean like Foxtrot just said, that whatever was kept there was not to be taken off premises.

Then again, everything from cigarettes to cleaning supplies, to most patient medications could be poisonous if used incorrectly, so internal use could mean just about anything that could be used in the facility that they didn't want to have in an unsecure storage anywhere where the patients or other unauthorized person could get access to it.
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Yeah, Cane Hill and West Park near Epsom were two places I was not able to photograph :(
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Actually, it's an anagram for 'Flat Sweaty Fly'
It's almost two different buildings.
Amazing little doorframe.
that's a rather attractive fire exit.
those blasted ceiling tiles never seem to stay on.
anyone ever been hit in the head with one of those? It's all pretty comical, actually.
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Wow, incredible view. Awesome shot.
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Was that grass cut?
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LOL BritChick!

Very nice picture!
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Holy shootipoop! Excellent shot!
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Oooooooh.......my.........GOD~!

*is utterly speachless*
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Pretty much love it... nice job!
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Oooh Mr Motts, you mention in your write up that you had probs with dogs at Cane Hill? Would that be Cane Hill Asylum in Surrey and are there piccies? *that place is about 20 minutes down the road from... where I used to live*
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Nice and creepy looking that, would be great to see how it looked at nightfall