I found this happily eating my lemon tree in my conservatory. The tree has spent the last 8 years in my N Cambs garden in summer and inside for winter, so it should be indigenous. Its 3cm longand I wondered if it could be an Black banded or Suare Spotted Clay.
I think both of those are unlikely, certainly in Cambs.
My guess would be angle shades, Phlogophora meticulosa.
The larvae can be green or brown, but they always have the little vague diagonal lines along the sides, just like yours. http://www.schmetterling-raupe.de/art/meticulosa.htm
However, there may be other moth larvae with that aspect to their markings.
I am not aware of angle shades larvae eating citrus, but some moth larvae are not very fussy eaters!
You will be able to compare your actual larva with photos of angle shades larvae better than I can.
Please let us know what you think.