Gouli - are you sure about your radar theory? The minimum distance for the radar on the Sea King was 75m which is why it was so good at coastal and vessel letdowns.
The min range is about 0.1Nm (from my memory) in SEA mode. In ground mapping, ehhh, I'd say around 0.15. It is a bigger value in WX mode, that'll be aound the numbers given by Gouli.
Gouli, its a 10kW radar, with 8kW as nominal power.
For others, about the radar painted picture: the screen will only show where the radar pulse hit somethig. From low level towards a very steep high hill higer then your height the closest part painted on screen will be the first hit close to sea, and the furtest will be the upper part of lobe hitting higher on the slope further away.( a vertical steep will only paint a thin line).
From wikipedia pictures, they flew towards this small island short side, that is quite steep. Wiki say Blackrock measures 400m length and 200m width, thats 0.1nm wide. So the radar picture will show a very small width( seen in track perspective). The length it will paint is the steep's horizontal difference + the "beach". From pictures that looks like a maximum of half length, ie also about 200m or 0.1nm.