Posted: 21/11/2016 at 13:56
I wouldn't water them at all in the winter - many Mediterranean plants can cope with low temperatures if their roots are almost dry, but cold and wet is very bad for them.
What sort of compost are they in? I would use John Innes loam based compost No 3, with added grit at a ratio of 3:1 to ensure really good drainage. Also keep the pots raised up on feet to enable the pots to drain well.
I would bubble wrap the pots really well, but I wouldn't fleece the top growth unless the temperatures drop to -5C or below.
Also, remember that even evergreen trees drop their leaves at some time during the year, they just don't drop them all at once in the autumn like deciduous trees. It may be that this is part of their normal behaviour, but it's being made more noticeable by having a degree of root rot due to the roots being too wet in the winter, causing them to struggle.