You don't live in cornwall do you...next door to me? lol
I think you will find it is of the Indian or Carausius Morosus variety.
I have just released about 100 into the wild. I had them as pets and I kept finding they breed too much and I could not collect enough ivy to feed them I thought they would be better out free in nature.
If your not my neighbour I think you will find that one of your neighbours has done the same thing as me.
You can buy stick insects kits as gifts you get a net habitat send off for some free eggs and then wait for them to hatch then they breed within a year, I had hundreds lol.
You can see the kits for sale here:
http://www.hawkin.com/find/keyword-is-stick+in sect/product-is-08929?SearchIsReferrer=true