Having seen the bird (thank you very much twaiyi) and done a little more research I am completely happy that this is Red-backed for the following reasons:
Tail structure: very short distance between shortest and longest tail feathers and very square-cut tail. On Brown the tail is more rounded with a greater difference between the shortest and longest tail feathers, in addition the send outermost is obviously shorter than the longest which is not the case with Red-backed
Primary projection: about 75% of the exposed tertials, but about 60% on Brown.
Wing formula: The second primary tips falls between the fourth and fifth primaries which is spot on for Red-backed, on Brown the second primary tip typically falls between the fifth and sixth primaries.
Bill structure: rather delicate and not as deep or strongly curved as on Brown Shrike.
Pattern of greater coverts: the broad pale fringe with a blackish sub-terminal line and a pale centre is typical of Red-backed. On Brown the greater coverts are extremely dark and with a broad rufous fringe.
Pattern of the tertials: pretty much as per the greater coverts for both species, but especially so on the shortest tertial.
Underparts: Whitish ground colour with neat black crescents; on Brown the underparts would be sullied buff and would be darker on the flanks.
Upperparts: slightly browner and more rufescent than Brown. When I saw the bird it was nothing like as rufescent as it looks on the posted photos and looked very similar to a first winter nominate cristatus at times but not so dark.
