hi the leaves on my tomato plants are tuning yellow any advise

It could be a number of things from disease,over feeding and or starvation plus old age.

A picture would be handy.

Meanwhile have a read of this;

http://www.thegardenersalmanac.co.uk/Data/Tomato-Leaf%20problems/Tomato%20-%20Leaf%20Problems.htm

Are they the bottom leaves below or near the first trusses?
I tend to remove these so that the plants get some air around them and it helps to stop any disease.
If its all over the plants and you have been feeding them as normal then you could try a dose of epsom salts to bring them into balance a bit

It's perfectly NORMAL for the leaves of tomatoes to turn yellow from the bottom of the plant as it grows. You only need worry in case the upper and newer leaves start to go yellow or brown, in which case it can be either blight or some lack of nutrition. Remove any dead leaves as it will let more air and light around the plants. Keep feeding with a liquid manure like comfrey or nettle leaves soaked for a week in water, as this will keep the plants healthy and going strong.

thanks very much for your comments. its probably lack of sunshine as in Glasgow we've had very little this year. i'll try what you suggest .