Hi everyone,
My orchid has had some brown patterns on one of its leaves for about one year. The rest of the orchid lookes healthy to me and tends to flower once a year when the days get shorter in the late autumn.
Looking through the internet I am not sure whether this is a fungus or a normal variant or excess sunlight since the other leaves never 'got infected'.
Please help.

@BigBill, @Ursula - any thoughts?

Would this be sunburnt or temperature-related damage? All the other growths look clean.

So I have changed this plant's position from a well lit area last year with some direct sunlight in the afternoon (baring in mind it is the UK the sun is seldom very strong) to an east facing window protected by a white see-through curtain so it gets a little sunlight in the morning but then indirect sunlight in the afternoon. The window is much cooler in temperature than the rest of the house and more humid. I can post another picture also of the other side of the leaf for any experts out there! Much appreciated. P

Your orchid is some type of Oncidium. Oncidiums need a lot of water and a couple of hours of morning sun, and then bright for the remainder of the day!
Your orchid is not getting enough water!!! The media, bark mix, is too coarse. It needs to have fine particle size so it can hold more water. Compared to many orchids that have robust, thick roots, the Oncidiums have very skinny roots. They need a fine mix to grow in. Adequate moisture will make the entire plant grow larger but when it grows with a water deficiency, it just limps along never really looking healthy. Those bulbs should get to be almost as big as a tennis ball and produce multiple flower spikes per bulb.
Fertilize half strength every other month.
Lastly, Oncidiums do not hold onto their leaves for ever, 2 years maximum so you can't expect leaves to look pristine for ever. They naturally yellow and fall off. It is normal.
Their leaves have a thin substance to them and are easily subjected to false spider mite damage. Those damages can lead to fungal spots on the leaves.

What Bill said!

Watering more regularely...the yellow spotted leaf has dropped off....

Pier, Oncidiums tend to hold leaves for 2 years maximum no matter how great the culture is. Relax.

Generally, once leaves turn yellow the main nutrients have fled and the leaf will eventually drop off regardless. That leaf was going to go, but the plant is looking good and I can see some new growths coming along.