Took Elmo's advice and added some 2mm pellets to my mix and compared it against the powder only and it wasn't conclusive and they still take an age to break apart. I added some 6 and 8mm pellets and that didn't really improve matters. That said, the Baking Powder was doing a really good job and the water in my tub was very murky that was making it difficult to see the bottom.
Several years ago I used to barbel fish a brilliant swim. There was a gravel hump about 15 foot from the bank and you could wade it with wellies. Just below the hump, the river shelved off steeply to form a hole. I used to stand on the hump and fish with the rod pointing directly downstream. If things were quiet, I would stub my toe into the gravel so it kicked off a stream of silt. The amount of times I had a bite shortly after couldn't have been a co-incidence and I caught many doubles to over 12lb.
Sometimes it's just doing something different to the crowds that brings results.