I agree that you should trim the top and sides because this does make for bushier growth. Your hedge may be suffering a bit from being planted in the same place as ther previous conifer hedge, which will have impoverished the soil, so a good feed will help it to grow more quickly. I wouldn't worry too much as it will fill out eventually and as yew grows even from old wood you can't really do much harm whatever you try. In my experience a well-nourished yew hedge will grow quite fast and soon provide an effective barrier unless you have heavy, wet soil which it really does not like.
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