Purple spray is next to useless these days compared to what it was.
Thrush is an anaerobic bacteria so exposure to the oxygen kills it, You need to keep your horses feet picked out and kept dry, I would recommend getting a wire brush like this:http://www.stromsholm.co.uk/heavy-wire-brush-5-row-1105-p.asp to properly clean out the feet and then treat with something like Kevin Bacon Hoof Solution, Hippo Sol or White Lightning
I personally use the Kevin Bacon Hoof Solution to clear anything up and then treat regularly with Swan Anti Bac Hoof Dressing to keep at bay. Hope this helps
I have for a while now noticed a correlation between thrush and what we feed. I don't know the possible reasons but since we changed to feeding a forage/concentrate mix we barely get any instances of thrush. You need to get a hose and with warm water thoroughly wash out right into the offending area, 2 or 3 times a day initially then spray with a good disinfectant using a plastic syringe deep into the gap holding the hoof horizontal so it has time to soak a bit. I have from time to time successfully used a bit of cotton wool soaked in disinfectant right into the gap if the horse is turned out on soft ground. Don't go overboard with disinfectant as anything strong enough will also impair the growth of the new tissue.
My Haffy is prone to thrush in the central sulcus. I treat it with cotton wool soaked in iodine, pushed into the sulcus. It works really well and you know the sulcus is starting to get better when the cotton wool no longer stays in the deep crevice.