I've just got back from Lichfield Record Office.
The 18 December 1801 marriage for Peter and Hannah Johnson - well, you have the entry.
But the venue was St Peter in Stoke on Trent, not St John in Hanley.
I checked baptisms at St Peter in Stoke from 1790-1810 and there are no entries for the surname Stephan. I double-checked with the transcribed baptisms. This was the only Anglican Church in SoT at that time.
The only other thought is either non-conformist or Roman Catholic, neither of which are at Lichfield.
I did note that the 1818 marriage is by Licence - it might be interesting to see if the Bond and Allegation have survived, and I believe these would be held at Hereford Records Office. I've found that they invariably have an age for bride and groom, apart from being nice to have 
Oh thank you ever so much and yes you're quite right about the church/location of the marriage - thats just me being dopey. I do have it recorded right, it's just that someone suggested St John Hanley to me previously.
They sometimes spelt the surname Strephon too but I'm sure you would have noticed that if you looked through from 1790-1810. Thank you ever so much for doing that, I really appreciate it.
Peter Stephan Snr whom married Hannah was actually Pierre Stephan and was thought to be french-swiss - Do you think it likely therefore that they may be registered as non-conformist births? - I assume they would be held at Staffs or Stoke.
Interesting to learn about the 1818 marriage for Peter Jnr being by license; might be able to get some more info on that. Do you mind me asking why it would be held at Hereford and not the main Shropshire Archives? I'm in Shrewsbury ya see.