I've been getting these for a while on repeat private precription from my GP, before Pfizer's patent ran out I used to be paying £60 for 8 tabs of 100mg (which I then cut in half). Now that generic alternatives are available, I have found Tesco's pharmacy the best place for these, I now have a presciprion for 16 x 50mg and it cost me just over £2, although the price has increased recently to just under £5.
Dose wise, 25mg is often sufficient, on empty stomach and straight into action - although I normally take 50mg. Taking any higher is counter-productive, doesn't make it any "harder", and if a girl doesn't turn me on nothing is going to work anyway. Paracetamol is good at countering the side effects of headache, but normally take that afterwards as it can also deaden sensation.
Just my 2 cents. Happy Xmas everyone!