Sad looking pieris with yellow leaves - what can we do?

~it might need more drastic help~could you do a repot with ericaceous compost?

It does sound like it might be in the wrong compost. Not being in ericaceous is a prime reason for yellowing leaves. You can get special feeds for acid loving plants. Miracid is one well known one although sequestrene will be fine.

Usual set of questions, is the pot big enough, are you watering and feeding regularly, when did you last repot, was the shrub protected from frost? If you have had in the same pot for several years it probably needs repotting into a larger pot with new ericaceous compost. Keep well watered after repotting and in a partially shady area to give hte Pieris time to recover.