United Kingdom 
My cucumber leaves and tomato leaves are al turning yellow and dying, although they still seem to be flowering and ready to produce fruit
29 May, 2011
Answers
There is also the danger of overwatering which will also turn toms yellow, a drink of Epsom Salts is worth trying for reviving yellowing plants, old-fashioned remedy....
29 May, 2011
Are they inside or outside?
I would not recommend watering as that may have caused the problem. Overwatering just washes away the nutrients, particularly nitrogen which may be the problem, especially with cucumbers.
29 May, 2011
If the tomatoes and cucumbers are in containers it is almost impossible to overwater.
30 May, 2011
And if they are in containers, what compost has been used. Not difficult to wash away nutrients if there aren't many there in the first place, especially these reduced peat/peat free ones that have different watering requirements.
30 May, 2011
Where are you in the UK Geoff, please amend your profile to show this, makes answering your Q's easier.
If you are in the drought ridden south I wonder if you have been watering well enough. Remove the obviously dead leaves but get watering.
29 May, 2011