Chlorosis. Feed with a special ericaceous fertiliser. Those leaves are dead, so remove them.
One other possible cause is over watering, so check that the compost is not saturated. Water with rain water only.

Many thanks for this information. I always use rain water and the pot is a mix of ericaceous and compost. Off to remove the yellow leaves soon!

After a while it will become pot-bound and there will only be roots with little soil left in the pot. With regular watering, all the nutrients will be quickly washed away. Perhaps a larger container or planted in the ground if you soil is not alkali.

Jimmy, I garden on Sussex clay so I think moving it to a bigger pot is going to be the way forward.

I have a camellia about 8ft by 6ft been in a very large plastic pot/tub for over 20 years and has been pruned once. I planted it about half way down the pot in John Innes Ericaceous Compost, Since I have put a bit if mulch on top every year (and always all the flowers) and only fed it once or twice during the 20 years, I have only watered a bit in summer when very dry and especially in August, I let it manage other times. Its in the wrong place facing east but by the house to block next doors ugly extension. Sun and light frost flowers ok hard frost and no sun flowers frosted. The compost is now about 3 inches from top of tub. Scented Red is the variety, I think you may have watered too much and pot too small.I am a bit west of you,

Jennh, thanks for your comment. I fed it with an ericaceous feed yesterday but shall leave it in that pot while the buds are there. Perhaps when the flowers have bloomed and gone, it will find a new and larger pot!