Hello there. I realise that I'm probably going a bit soft and this is just a normal Oak Bush Cricket that I found hiding beneath a Willow Herb leaf but it seems somewhat less robust and rather more delicate than our normal local crickets that I had assumed were of the Oak Bush variety. This one was a strong flyer with a body length (including ovipositor) of 25mm and a further 25mm of antenae. Could someone please confirm the ID.
You were correct with your first thought, the Oak Bush-cricket. See the 'Species of the day' link below for the Southern Oak Bush-cricket, which though very similar in colour, is brachypterous so flightless.