Update: I didn't have time to trial run it so went for a 30 min punt as my trial.
I had a pint of water, 2 x paracetamol and 25mg Sildenafil 40 mins before my punt.
I felt no different, I didn't flush, feel dizzy, headache etc.. Thought it might have been a dud.
I went on the punt, soon as she put it in her mouth I got an iron bar. Not had one that hard in ages, it was like a morning wood hard.
It's amazing stuff!! Thanks for the help folks