Ioannis,
You're right, ~7k @ 5V Vdd - I totally missed that graph!
As you say, a longer sampling period may be needed. Trying will tell. Also, the PIC Mid Range Manual sugests a 0.1uF capacitor on the input if the signal isn't changing to quickly, may be worth a try if Sam decides to use the circuit.

Thanks for the heads up on that graph!

/Henrik Olsson.