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Very hard to tell without seeing the tree. Does it show any white spots where the bark was lost or are there any yellow leaves around the affected area? One of the causes for loosing the bark could be inconsistent watering, but you have done this on a regular basis. So it is a bit of a mystery.  

 Gala, thank you for your response.There are no white spots or oozing.  Since my post I have discovered on an Australian website reference to this happening due to sun burn.  It states that lemon tree bark is not very resistant to sun and need protecting, the site refers to painting the bark with a special lemon tree preparation - not seen anything other than food for citrus trees here in Italy.  Have you heard of this before?  As an immediate measure we have moved the tree from its south facing position to a position where it only gets full sun in the late afternoon.  Not sure what to do about the existing damage - I feel like I need to "soothe" it with after sun and wrap up the split branches with a bandage :)  We visited our local agricentre this morning and they just said to ensure we don't wet the trunk of the tree when feeding it.  I don't think the lady we spoke to entirely understood the problem (my �������� and her trying to serve 3 people at the same time).As a separate issue (well I assume it is separate) on closely inspecting the whole tree we noticed what appear little white nests (have lots of little eggs inside).  These are white and black, less than 1cm in diameter and are nowhere near the affected bark/limbs, these are on younger wood higher in the true and in total we found less than 12.  We have hand picked these off and will keep close watch to see if more appear.

 Citrus trees are prone to to white wax scale when the warmer weather sets in. There are special sprays for that. Does it look like this?  As for the sun, the Australian conditions are extremely hard. I lived there. Lemon trees like sunny positions and I do not think that you would have a situation similar to the Australian one in Le Marche. I have not seen a special oil for Citrus trees sold in Italy. P.S. I could not insert the photo, but have a look through Google and you would find several photos of the damage caused by white scale. 

Yes, odd critters lemon trees, one of mine has been in a pot for ages and has suffered from split bark, shedding of leaves etc and I have recently replanted it in a tile-space on the terrace and at last we are seeing a big improvement! but as it is only 2 foot high there's a bit of a way to go yet!