There is a plant identification (Plant ID) forum here on this site, your thread could be moved there using the "Suggest a Change" option under the latest post in the thread. Identifying the plant is the first step in identifying a problem if there isn't an obvious culprit.
If you do find you need to ask on a UK forum, the RHS has online forums and you do not have to be an RHS member to participate. If you are an RHS member, you can send their diagostic service the pictures of your problem plant and they will tell you what they think it is.
One thing you could do for here is shake a branch over a piece of white paper and see if any insects fall onto it, which makes them easier to see.