Trouble with male dogs is that they make me ask embarrassing questions, my male dog isnt particularly highly sexed but gets an erection when he lays quietly (sometimes after washing himself, sometimes not)  and then lies there in pain when I make him walk a few steps he goes back to the 'normal' size, he isnt enlarged normally and his testicles arent enlarged, think it's his age as he's nearly 3 and this is really recent, do I need to get this checked by a vet or is he a normal healthy boy and I dont really want to have him neutered unless I have to, would something like scullcap & valerian help ?

The only aspect of your post that would worry me is your assertion that he is in pain - that part is unusual.  Two things that might be causing discomfort are 1) that he's leaving his penis out for a little too long and it's getting dry which is then making it difficult to retract or 2) as the penis retracts it's folding some hair with it which can be very uncomfortable.  You can help the former by using some ky jelly, or just getting to him quickly if your intervention makes him retract it sooner and you can trim the hair away to prevent the latter.

I have two entire boys age 5 and 2.  The causes and the timings vary, I've seen it when they pass a hard stool for instance, or it can be general excitement, it can also be part of their washing ritual but it's an involuntary reflex response - one of my dogs when a puppy used to be terribly surprised by it's appearance and give me a "uh oh, what's that?!" sort of look :)  I've also found that these kind of habits and behaviours happen for a short period of time and then simply go away.  In my experience it happens more frequently when they are young so it could well be hormonal but I don't think there is any need to consider neutering - it's something that affects neutered boys too.

I have a 18 month old full male  GSD that has this problem on waking from a nap ,looks really shocked ,as he walks around with it he looks like a cat with a arched back,couple of minutes & hes back to normal ,he doesnt ejeculate ,he has been around a bitch in the last week of her season & taken no notice of her ,apart from here's my friend to play with !!!! & it dosent happen then .

I've got an entire male that does this when he is bringing me his teddy, and I speak in a stupid high pitched voice.  He gets very excited and wags his tail like mad, but I honestly don't think it's sexual.