With Virgin's Trainline.com the earliest ticket that didn't cost a hundred plus was 6.40 PM - came out costing 43 quid. It's a direct journey, no changes, but I really didn't want to wait around for two hours and arrive in Wolverhampton around nine. And the return journeys offered were nightmares, with two changes of train.
Fortunately I remembered that I'd travelled from Marylebone once and had a reasonably good journey. So I looked up Marylebone and found it was run by Chiltern railways, hit their site and specified that I wanted to go from Marylebone (which happens to be the closest mainline station to where I work). The ticket came out at £35.60, and I could get the 4.30 train. The only drawback is that I have to change stations, not just trains, in Birmingham, and it's about a 5 minute walk from one to the other. The journey back is even simpler, just one change of trains at a single station. So I'm paying nearly ten pounds less to arrive in Wolverhampton two hours earlier.
Problem solved - but it's a disgrace that there isn't a central booking site for all of Britain's railways that offers alternatives like this without having to hunt for them.