nutrient problem / yellow patches on leaves

Hello!
could someone please help me and let me know which
nutrient my plants are lacking? the one pictured has a couple of yellow blotches on its large leaves as they are on a couple of other plants, apart from that they look a healthy shade of green, new leaves are just sprouting and they have no yellow at all.

One plant in particular, (which isnt pictured) its lower leaves are yellowing so much they are wilting too?

i know its a poor pic but hopefully u can still make it out, any help help would be much appreciated,

thanks
http://www.freefilehosting.co.uk/fil...0003%20(1).jpg

im going to get some better pics and i know i should ph test the soil before adding a nutrient like magnesium to tray and fix it but i have a ph soil test tube kit from a garden centre which is cheap and last time i tried the same one it didnt work? ive got some epsom salts, fish, blood and bone, dried blood and super phosphate ready to add in but i have never added these to my plants yet. The only nutrient feed ive givin them is bio bizz biogrow 0.5 ml the 1st week, 1 ml the 2nd, 1.5 the 3rd and planning on adding 2ml for the 4th which is due today. So am i rightin thinking it wont be a nitrogen deficieny as this is provided for but may be magnesium as its not included?