insects on cucumbers

Hi, I have some cucumber plants in the greenhouse, which so far have yielded well. However the leaves  are very mottled and yellowing.

I assumed red spider mite.  I can see a few tiny insects on the underside of the leaves but I thought spider mite would look like pin heads or tiny dots. These are tiny, minute in fact, but look like little fine pencil line shaped, that is, elongated rather than roundish. They move quite quickly on the leaf if disturbed.

Can anyone confirm that these are likely to be spider mite or are they the wrong shape?

Thanks , Could overwatering cause this defficiency if the compost is waterlogged?

Thanks Tony. I have just looked in my tomato book and it says too much Potassium can lead to magnesium shortage. The plot thickens. Out of lazyness I use the same water for my cucs as my toms which includes a daily feed of Chempak Tom feed, Maybe you have hit the nail on the head!

I will try an epsom salt spray.

Thanks.

A daily dose of tom feed seems far too frequent - is this the dosage rate on the bottle / box?