Without testing the soil, you won't know the ph, but you're right, Skimmias prefer acid conditions. If people in the local area are growing Pieris, Azaleas and Rhododendrons and blue hydrangeas in the ground successfully, your soil is on the acid side. If you're concerned, or the yellowing gets any worse, you may have to resort to using Sequestrene iron tonic every six months to keep the conditions at the roots more acidic. Other thing to remember is that Skimmias keep their colour better in shadier conditions, they tend to look quite yellowy green when in full sun all the time. Also watering - shortage of water could cause yellowing at the tips too, which will progress to browning.

Thank you Bamboo :)

I am going down to the garden centre today for a ph meter.
I do presume that the soil is on the alkaline, since my dwarf rhododendron is also taking ill :(

Im not doing too good at this!

but thank you for your advice :)

If your soil is alkaline, might be better to restrict the acid lovers to pots only and use ericaceous compost for them.