Who or what is eating my Leeks?

Weather: Cloudy but quite bright. Getting colder. Daytime temperature 3C

I've grown leeks for many years and they were one of the few of last year's successes in that they didn't appeal to my burgeoning colony of slugs. But now something is finding them tasty and I'm pretty sure the culprit is rather larger than a slug! The garden is virtually enclosed by a stone wall and I've never been troubled by rabbits so it's a mystery.

Fortunately whatever it is it seems satisfied by the outer leaves at the moment and the bits I want to cook remain intact.

Recipe: this is my favourite leek recipe and it is really easy - it is called Wrexham Bake although the reason for that is unknown.

Ingredients:
1lb sausages
1lb leeks - (cleaned and cut into 2 inch lengths)
1 15oz can of chopped tomatoes
A large eating apple (cored and chopped into large chunks)
Salt and Pepper (I also like to add a few pinches of dried chili)

Method:
Put all the ingredients in a casserole and bake in the oven at about 200C for 45 minutes to an hour. You can brown the sausages first if you prefer.

Day two and I'm already digressing to recipes!