About 30cm long. I believe the dark flanks indicate that it's a female.
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You are definitely correct this is a female. The dark flanks are a good feature, although occasionally you get animals which aren't as clear as in your photo. From your photo you can clearly see this animal has a relatively very small head which is s great feature for sexing slow worms. A male's head would be relatively much larger.
Lat/Lng: 51.38518, -1.27297
OS grid ref: SU506653