Manic Chiffchaff (tentatively identified as species tristis (Siberian Chiffchaff) due to large size, paleness) travelling up the Cynon. Brief glimpse in the distance of a gleaming white floating oval on edge of lake could only mean a Little Egret or a rugby ball, time I got back with camera was spooked. Also Otter, Kingfisher, Mute Swan, Dipper, Sparrowhawk.
Glamorgan Bird Club 
Did you hear it call? This is a rather pale chiff!
This is indeed a very grey looking Chiffie – poss tristis.
Hoping you might have agreed with my initial thoughts! Yes I thought it looked a little large for a Chiffchaff, and the paleness of the chest was striking, plus was a lot more manic than a chiff. No call, and probs never seen again. I suppose I shall have to wait for your official decision on it!