Parking is available at the location (small cost) and there is a café and toilets available. See here: https://www.swansea.gov.uk/article/6873/Lliw-Reservoirs---Upper-and-Lower for full details.
The property is technically owned by Dŵr Cymru [Welsh Water] and whilst I can't find any specific advice to this site, there is, I believe, a general acceptance to the flying of drones by them so long as one follows all current advice and legislation. Just don't be like the idiot I saw on my visit that flew from the car park!
Land owner permission requirements unknown.
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Co-ordinates: 51.71312, -3.957669 • what3words: ///lemons.employ.fatigued