iron chelate, sequestrine, any garden centre stocks acid loving plant feeds.

Sequestrene, as bizzby mentions - its an iron tonic, sounds like your soil is alkaline rather than acid, and rhododendrons hate alkaline soil. Water the sequestrene round the base twice a year, or dig it up, put it in a pot with ericaceous compost.

Most of Aberdeenshire has acidic soil... I wonder if this isn't the stress of the long very hard winter we had up here.

Well that's interesting MG, perhaps its not iron that's the problem. Guest, if your yellowing leaves are going completely yellow, that's iron shortage, but if they're going yellow between veins, that's magnesium deficiency.

Guest join GoY then we can have a two-way discussion... it's free and we don't bite!