I think it depends to a great extent on what a "manager" role consists of - the job titles and descriptions aren't standard across the country, and it will vary from one company / group to another, and within any company it will depend on the size of the depot and scope of the job. Some depots may be little more than an outstation, others can be huge.
At some depots, the person in charge (whatever their job title is) is a front line supervisor and not a lot else, and the pay for that sort of role, as others have said is likely to be a bit more than a driver (and with possibly less scope for overtime) - it's fairly rare for this level of job to be advertised externally, most companies tend to promote from within for this sort of level.
At some depots then the depot manager is the one making hiring / firing decisions, commercial decisions about routes / timetables and so on, and may oversee the engineering function as well. Again, some of the bigger groups rarely advertise this sort of role externally, especially if they have a big graduate training scheme.
All I can suggest is keep an eye on the jobs pages in "route one" and the other industry press, and "jobs in transport" website - a quick look today doesn't show any "depot manager" type jobs that show a salary.