Thanks for that.
The article, APOTHECARIES' HALL . The Standard (London, England), Friday, August 18, 1854; Issue 9370, seems to report on him completing his examination in Science and Practical Medicine, receiving a Certificate to Practice Medicine, but it refers to him as Evan Pierce Williams, Denbigh. This is a couple of years before he's appointed medical officer.
In an article in ST. ASAPH . North Wales Chronicle (Bangor, Wales), Saturday, December 15, 1860; Issue 1757. he is again referred to as Evan Pierce Williams, Denbigh. Infact I think in this article he's actually introducing himself as such.
I suppose his mother might be a Williams but seems slightly backward for the time to put his mothers maiden name after his father's surname.
Interestingly he's not living at Plas Meifod in 1841, and he doesn't seem to show up in any census records there, so, where is he? Is he next door?
Also, 'plas meifod has been mistranscribed in the census records a number of times. There's Plas Morfod, Plas Minfod and Plas Merford.
Edit: Oh I think this explains it. Turns out there's two of them.
Deputy coroner [no ref. or date]
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Appointment of Dr Evan Pierce Williams, Vale Street, Denbigh, as deputy coroner for the Denbigh Division to Dr Evan Pierce of Denbigh. QSD/DO/4/1 1857
http://apps.nationalarchives.gov.uk/a2a/records.aspx?cat=209-qs_2&cid=1-12-4#1-12-4Wonder if they're related.