If it is intermittent in either position then favourite is:
Either the small wire that goes to the starter that needs a clean on the connection.
Or:
The starter solenoid has worn out, very common problem, replacements can be had from Ebay for around £15/ £20. Make sure you get the correct one for your engine gearbox combination.
Allow around an hour to hour and half to fit it, take your time.
Some great How to's on here to help you do it