This one is going to be impossible! I would love someone to prove me wrong.....
I have a few types of grasshopper which I think I have managed to ID, there are a lot of forms even of the same species in some cases, but this one seems to be virtually non-existant.
I have searched the main sites which show variations and species, including bio-images,
http://www.fugleognatur.dk/english/art_uk.asp?mode=liste&id=10
There is one on David Element's site which comes close, BROWN-SPOTTED BUSH CRICKET Platycleis tesselata, but is a poor pic and it's not supposed to be here, the write-up at the bottom says it is moving northwards and could be in the future. When that was written I don't know.
http://www.david.element.ukgateway.net/grasshoppersandcrickets2bushcrickets1.htm
The only other thing I found is on delta-intkey, a description of Gryllus bipunctatus
http://delta-intkey.com/britin/images/text439.gif
this sounds very much like it, if you read the italics description many details fit mine, including the groove down the face.
I found a list of species supposed to be here, this is not included.
http://data.gbif.org/species/browse/resource/863
This German site has some drawings of other Gryllus species which show similarities, scroll down to the list,
http://caliban.mpiz-koeln.mpg.de/~stueber/panzer1/index.html
This pic shows the groove down the face,
