Plant foliage turning yellow

Was the soil changed when you did the walled garden? I'm wondering if the soil became more alkaline, as skimmias need acid soil, and alkaline conditions would give them chlorosis. If you're in a hard water area it may not like being watered with tap water either because of the lime in it - water from a water butt would be a safer bet.

If it has chlorosis the veins will remain green while the rest of the leaf goes yellow - does it look like that? If so it might be worth feeding the plant with some kind of ericaceous plant food so that it gets the iron it needs. It might be worth a go anyway just to give it a boost but don't give it more than the instructions state.

Is it waterlogged? Too much water is just as bad as too little

Dont think your Skimmia will have appreciated a hard prune.Flowers and berries produced off new growth shoots.This explains how these are kept to the outer edge of the plant each year.

Ellemes comments quite right and heavy periods of rain can actually wash away soil nurients which confusing I know as people often say cant understand why leaves yellowing when never allowed to dry out.

I use liquid tomato feed for them.Think the prune could be terminal as most growth tips will have been removed and not sure new shoots will develop off remaining tips.