Rusty red tailed, grey, robin-like bird fliting from piles of rubble, brash and wood. Not showing any white patches. Vibrating tail on landing. Seen on Rhiwbina Farm, Rhiwbina Hill ST145829. If heading north, look over road side field gate on left on to farm material piles. A right of way goes through the field gate SW then W to Tongwynlias. Seen on 17 April, 2017 at 11.35am.
Glamorgan Bird Club 
More likely to be Common Restart in this location IMO – description sounds like female. Apologies if you are familiar with both speceis, if not, check images here, for example: http://www.birddb.co.uk/findpicture.php?common_name=Gartenrotschwanz&lang=de
Sorry to be a while replying. Yep thought the same myself at first. Did have good views for a couple of minutes but no sign of the slightest bit of pale underneath ( or buff) which made me think it was a BR