Ailing hibiscus

I have a well established hibiscus oiseau bleu but for the past two or three years it's leaves have yellowed as the summer has progressed.  I have used seaweed feed with a bit of success (new leaves forming nearer the base come out in with a healthy green) and it flowers as well as usual.  I wondered if it is getting too much competition from my neighbours nearby large shrubs.  My soil is slightly alkaline clay, which I have improved over the years.  I mulch the shrub with leaf mould each autumn and with my own compost in spring.  I don't want to get rid of my hibiscus but it doesn't look very attractive.  I also can't decide wether it is getting too much or too little water.  It's south facing 

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