Hi John,
Thanks for the information. I'll persevere with the warblers and hopefully will start to recognise the calls in time.
Just looked at some images for the Black-winged Cuckoo shrike and the head, body and beak shape certainly look better proportioned. Unfortunately, due to the quality of my photo, it's difficult to see the markings and the plumage as the image looks rather dark.
Thanks once again for your help and advice.
Andrew