I have a staffie dog now aged 18 months, he has always been lively and played rough as staffies do, and was very very difficult to stop play biting in the early months.  However, yesterday he ran upstairs and got on my bed when I was about to change the sheets and when I told him to get off the bed he didn't move so I gave him a gentle nudge and told him to get off.  To my surprise he jumped up at me and grabbed my arm in protest, using his teeth but without biting if you know what I mean.  I should mention that he does sleep on the bottom of my bed at night. I have the feeling he thinks he can get his own way now by challenging me in this way and I want to stop it immediately before it becomes a real issue.  Would very much appreciate any advice as although I have had staffies before none of them have ever tried it on in this way.

if it were me i wouldn't allow him on the bed but if you want him to sleep with you then give him a nice bed of his own in your room.
a lot of dogs can get funny about furniture as they seem to think it's theirs.
i would also stop him getting on the sofa if he does that as well. putting these boundaries in place will help define what is yours and he will learn his place better.
if your worried that training him to stop doing this may course problems then put a light weight lead on him in the house and if he want get off the furniture then use this to gently pull him off. if he does this with no fuss then reward him with a treat. this can take a while to stop as it's a habit but just keep going. once you've managed this never let him on the bed again as all your hard work will be undone.
i have a schnauzer that got silly when allowed on the sofa so i stopped it before i got my third dog and she has been a lot nicer about everything.  
of course you may not want to do and maybe someone else will have a different idea for you.

18 months old.. I'd say he's pushing his luck as a teenage to see what he can get away with.  Don't let him!  He's backchatting basically (in dog terms of course)He's a dog, he needs to know his place.
If he's a typical Stafford, then he could be quite food orientated.  Re-set and re-train his boundaries.

Kevin syndrome as in Keven and Perry we call it lol!
He is deffo trying it on and simply using a strong voice, a sharp command and taking no liberties is the way forward.
my boy will still try to stay on our bed sometimes, I ask him off and he gives me the cheekiest look before having a twirl and looking at me as if to say 'yeah right!', I then straighten up, use a loud 'off' with a finger poinbting to the floor and hes off in a flash, my boy will be 2 in March as well