W210 E320 almost won't start puzzle.

[Graham]

Went out for gas & car wash. Car started normally. Drove to gas station.

- Filled up (100%) and went to re-start.
- Engine barely turned, started but stopped when I took foot off pedal.
- Tried again, barely turned over.
- Fiddled with key and gear shift to make sure everything OK - Tried again - barely turns over.
- Turned key off and tried again - nothing - engine does not turn over.
- Again fiddled with key and gear shift and tried again - this time engine turns slowly and catches - kept foot down until I was sure engine would not stop.
- Went through car wash and then drove home.
- Stopped car in garage, then restarted no problem
- Tried again no problem.
- left car for an hour or so (lunch!), then checked battery voltage (12.15v) and then restarted, again no problem.
- checked voltage while charging - 13.97v. (bit low?)
- Sealed battery so can't check cells.
- Battery is about 3 years old.
- Checked connections and also connector voltage drop - all OK.

Any sugestions as to what might have caused this?
Could of course be battery but why just at this one time and then OK again?
Could it be something to do with overfilling tank? - probably not because engine turned over slowly.