What is the easiest way of finding the maiden name of the wife, Susan.
There isn't anything easier than what you have done. Based on the 1881 Census, it's likely they married a year or two either side of 1868.
So a search reveals:
1. JAMES CLARK
Gender: Male Marriage: 19 NOV 1868 Barton In The Clay, Bedford, England
And looking that up reveals:
Marriages:
Spouse: SUSAN FROST
Marriage: 19 NOV 1868 Barton In The Clay, Bedford, England
That do?
What years were Census's taken. Is there a 1861 Census for instance.
Every 10 years. There was one in 1821 and in 1831 as I recall, but they didn't record names so aren't much use to you. I've waded through the films of the 1841 census at the FRC in London, and extracted bits from it. (Maybe the 1831 had names, can't remember at this distance, but it wasn't much help if it did cos there's nothing in my notes from it).
The 1881 is available on CDROM (I have it here, which is where the family details came from). The 1891 as far as I know isn't. The 1901 is available online, and you can look up all sorts of information, print it out, and so on (for a fee).
I therefore suspect that James would have attended the chapel I now attend, Hope Chapel in Barton le Clay.
I don't know what records (if any) chapels kept. Maybe the elders (or deacons, or whatever) will know? The Church of England has been legally required to keep records since 1550 or thenabouts, so if you can find where those records are stored, you can do a lot.