White blotches on tomato leaves

HI, have white blotches (not yellow) on my tomato leaves, general plant growth poor.  cultivation - ring culture method.  Any ideas? Two plants 60cm apart have the same problem.  Variety Gardner's Delight, grown indoors, soil medium 'grow bag' water from the tap.

Thanks Yorkie,  Watering through the soil as roots have not reached the gravel, there is leaf curl on some of the worst infected leaves.  Sorry no camera till Monday.

Cucumbers at least have the decency to just die if they don't like their growing conditions (or even if you just give them a funny look!)

Tomatoes can theoretically succumb to all sorts of nasty diseases and can have you Googling your butt off for "tomato leaf disease" pictures, but as often as not it's just a sign that they are suffering from persistent low temperatures and insufficient sun (aren't we all!) 

Thanks all, I have decided to scrap the plants and start again but this time not to use the 'grow bag' contents as soil in the ring culture pots as I think it was 'old stock' when I bought the grow bag judging by the concrete feel of the compost, with little 'goodness' left.

So my next question is - which JI grade should I use? and should I incorporate well rotted horse manure as well?