Looks like chlorosis. Could be over watering or nutrient deficiency. Yes, a layer of good rich mulch would help. Cut back on the watering - not too much.

Are the parts which are yellowed more exposed to sunlight? They're not too keen on strong sun, and they do yellow or go blotchy yellow if they're exposed to it. I doubt its iron deficiency, since its not affected the whole plant, but do you know which variety of Skimmia it is? S. japonica 'reevesiana' does not like alkaline soil, the other varieties aren't so fussy.

I'd be tempted to give it a dose of Epsom salts just to be on the safe side. but I suspect it could be due to too much sun. how old is it?

Oh thats saved me asking, mine has done the same so tomorrow I'll move it down into the shady part, thankyou all...Gotta love these pages, answers without having to even ask...

Bathgate - I will give it a mulch.

Bamboo - it's in a relatively shady place but it could be that the one side does get scorched by the sun. Can't say we've had many sunny days here! I've had it quite a few years , it started off as winter bedding for a hanging basket and don't recall it's variety.

Seaburngirl - excuse my ignorance what does Epson salt do exactly?

Linclass- glad I helped you with query! lol

Epsom salts provide the element magnesium which the plant uses to make chlorophyll [the green pigment that the plant uses in photosynthesis]

Thank you Seaburngirl. You learn something new every day!

I'd be very interested to hear how this goes on Amsterdam, if you remember. I had two skimmias in a north facing bed that this happened to, starting with one branch on one of them and slowly spreading over several years until both died. Nobody on Goy had any idea why - don't want to depress you and I hope yours doesn't go the same way. (I did try Epsom salts)