"He moved 10 minutes away a year ago and has continued to see then only a handful of times. He has taken a lot more interest in the last couple of weeks since he's been with his girlfriend."
Frankly, it's in no-one's interest for your ex to use your children to pretend to his new girlfriend that he is Father Of The Year. Because that is what he clearly intends to do.
It's not in your sons' interests to be paraded to new girlfriend and fawned over because it may mislead them into thinking their father gives a shit, and so the disappointment when truth dawns will be all the greater
.
It's not in the new girlfriend's interests to be misled into thinking her new boyfriend is a decent human, plus she'll be expected to do all the gruntwork.
It's not even in your ex's interests really, because girlfriend will wise up all the quicker to the scumbag he is (if she's paying attention).
"I agreed provided they would pick them up and drop them back, to keep the peace as he has been known to kick off at me if he doesn't get his way and also because I knew dc would appreciate seeing him."
Even if your sons would appreciate seeing him, as parents it is more important to protect them than to please them. And really, preventing him from kicking off was your primary reason. It's understandable, you're trying to do the right thing, by you and by your boys, and having to deal with an ex like him is not easy.
"I am now worrying this is going to become a regular thing. I am not prepared to do a 2 hour round trip when he has a place of his own 10 minutes away where he could have them, but also feel it isn't his girlfriends responsibility to taxi them around. Would that be really unfair of me? Should I be driving them any distance so they can see him?"
This is actually the easiest thing of all. You have no responsibility to ferry your sons to your ex's girlfriends home. Your responsibility is to make your sons available for an access visit to their dad. And please note, the access to their dad if for your sons' benefit, not their dad's. Whilst taking them 10 minutes to his would be very nice of you, travelling onwards from there to his girlfriend's house is totally his responsibility.
So no, it would not be unfair of you. And you should not be driving any distance demanded by your ex.