Reddish spots on leaves of a greenhouse tomato plant

It could be Magnesium deficiency although in my experience many leaf problems look very similar.

There are a number of possible reasons for magnesium deficiency and one of them is over feeding (although at once a week this is unlikely). Tomatoes take up potassium in preference to magnesium so a high potash feed (such as most tomato feeds) diluted incorrectly might prevent the plant taking up enough magnesium (which could be a trace element only in the soil).

I believe that if the plant is short of magnesium (which it particularly needs for new leaves), it will take magnesium from the older lower leaves and move it up the plant to the newer, growing leaves, giving the lower leaves the mottled appearance.

A solution is Epsom salts, misted onto the leaves when the sun is off the plant for a quick fix or watered around the plant for a longer term solution.

Finally, look at your tomato feed. Some just give NPK, others give NPK and a selection of trace minerals including magnesium.

I don't know where you are Boltardy - but we've definitely had fluctuating temperatures in Leeds!