Posted: 02/10/2015 at 15:03
Sounds like a combination of general hunger and specific chlorosis to me.
Re-pot in fresh ericaceous compost, water with some sequestered iron solution and give it a foliar feed by pouring over a solution made from 15ml of Epsom salts diluted in 5 litres of water. Repeat the foliar feed weekly till the frosts come and start again next spring if needed.
Every spring, replace the top couple of inches of compost with fresh and ad d a handful of slow release feed such as blood, fish and bone or pelleted chicken manure. Always water with rainwater or else tap water with added sequestered iron as this helps the roots take up nutrients that keep the plant healthy.