Not heard the Graham Slee but I own the 640p. It is fantastic value, even with the recent price hike from £60 to £80. A genuine hi-fi bargain. Sounds superb with my Pro-ject Xpression II turntable. Great rhythm, detail and superb timing. Far better than my old Audiolab 8000a which was noted for it's onboard phono stage and one of the reasons why I bought it..
A friend of mine has the Cambridge 540p which is a bit cheaper and is also excellent value. Doesn't have the mc input that the 640p has but is very close in performance to the 640p and is only £50.