A very small caterpillar 8-9mm long found on honeysuckle it looks dark green in colour.
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your suggestion of Lilac beauty is very likely to be correct as the habitat and available host foodplants of Ash,Wild Privet and Honeysuckle are in abundance in a 3 acre mature woodland where I found it .
I don't think it's Lilac Beauty - they have very visible humps on the back from the first instar (http://www.ukleps.org/geos/1910s.jpg), which this doesn't have. With the broad pale stripe below and pale flashes above, it looks rather like a Mottled Umber larva (http://www.ukleps.org/geos/1935s.jpg). The mature woodland setting would suit it too - they eat most deciduous trees - but it's difficult to say anything more than 'geometridae' with any degree of confidence!
Record your ladybird sightings!
At 8-9 mm the caterpillar has done a fair bit of eating. MU's dates for the larval stage are given at April to June. I know that dates published, as with Porter, 13 years ago, can now be inaccurate, I'm not sure that the conditions in 2009/10 winter will have accelerated things that much.
But, we are agreed on a geometer!
Foxy has added more pictures of this caterpillar at http://www.ispotnature.org/node/26552
Lat/Lng: 52.5, -7.8
OS grid ref: SF0814