HeadFairy I completely understand why you are not prepared to throw the towel in over this. But you do need to start putting down some pretty strict ground rules. Forget the couples counselling. Just tell him from now on how it will be.
He will come straight home from work
He will only go out drinking once a month
He absolutely will be on that last train, not spending fortunes on cabs
He absolutely will not be sending any texts to say he's 'crashing in town'.
He will agree to answer your calls/texts while he is out and not give you any crap about no signal, out of battery, couldn't hear the phone, etc.
Have a phone that rings and vibrates simultaneously, keep it on the loudest/highest settings and keep it in your pocket. How hard can that be?
(Just where does he crash exactly, and how much does that cost, anyway? 
)
Do you think it's likely he's developed a habit/addiction to lap dancing bars or prostitutes, rather than having an affair?
And perhaps it's time you suggested one of those phone tracker things, so you can know exactly where he is when he's out. If he works in the city and he's doing the 'few beers after work' think then he should have no need to be more than half a mile, a mile at most, away from his office or the station, should he?
I'm sure plenty of people disagree with those trackers in principle, but his reaction to you requesting one should tell you all you need to know on that score. If he really has nothing to hide and he wants to reassure you then he'll agree to it.
Or you don't lay down any ground rules, you let him carry on, and you get a private detective. That way you'll know pretty quickly what exactly you are dealing with here.
And start looking at his spending patterns on your bank statements. Is he drawing out unusual amounts of cash when he usually pays for everything on card? Does his credit card statement or bank statement corroborate whis whereabouts on his nights out? If he's crashing in hotels can you see the bills on his statement?