I don't want to contradict anyone here, because I think you've all had some great ideas, I'd just like to add my $.02.
From my experience, most dogs prefer to go to the bathroom in "privacy", they will go to a corner of the yard, or (if it's inside) another room to avoid having "company" while they do their business. We've had a few dogs at work who absolutely refuse to relive themselves on a leash, but will run to a corner and go almost immediately when loose in the pen. This could be part of the problem. Dudley may prefer privacy when doing his business, so he's waiting until you leave to relieve himself.
Also, naturally, dogs to not want to be near their own waste, so this may be why you have to "drag" Dudley over to the pile he's left on the floor for you.
Is it possible that he really doesn’t understand that going to the bathroom inside is wrong? His "guilty" demeanor when you return could simply be a reaction to you being upset. He knows that you are mad at something, so he gives you those "please don't be mad at me" eyes.
Anyway, getting to the point, my advice is this : First, make sure Dudley gets out to relieve himself after he has eaten. Give him a good 1/2 hour or more to digest, and then take him out. When he goes, make sure to praise him like crazy. As Renee said, this will help motivate him to hold his bowels until you return, so he can relish in the attention he receives from doing what you want.
Second, I would recommend getting a crate for when you have to leave Dudley alone. Since you said you were home most of the time, he wouldn't have to spend much time in it, but I think it might help. Crates are one of the best housebreaking aids. The confined area will help teach him where it is appropriate to go to the bathroom. Since he will do his best not to soil the crate (knowing he will then have to be stuck near it), he will hold it as long as he can. As long as he has had a chance to relieve himself before you leave, and he hasn't eaten too much, the crate should be clean when you get home (as long as it's not more than 8 hours later). As soon as you return, open the crate, and bring Dudley straight outside. Since he is left with no other option than to go to the bathroom outside, he will quickly adjust to the behavior you want.
I hope this helps. If you have any other questions, feel free to ask.