Roger... You won't like my reply
I was in a high wing a/c so I could only see him once he was in front of me
Errr... whenever I flew a "high wing" (152 etc.) it had windows out the back... so if I maintained a good lookout (which includes covering from nearly 6oc one side to the other) I would hope to have seen him approaching
At the time I was receiving a FIS but didn't receive any prior warning about the traffic
Not sure a FIS should have done so
The guy clearly wasn't in contact with any unit
And
What could I have done to prevent this?
An active lookout

I presume you were VFR
Would I have been better having a RIS?
Again, not sure a RIS has any guarantee that they will warn you, let alone give you avoiding action, of VFR traffic
Did I let him off too lightly?
What's it to do with him
Anyone had any similar experiences?
Most of my training, where failure to spot the "intruder" equalled a "failed trip", and in Operational Experience, probably equallled "dead"
As I said, a harsh reply... But if you are flying VFR, then a midair takes "2 to tango" i.e. each at fault. You cannot rely on the "system" or "the other guy" to avoid... you, and you alone, are responsible for the safety of your airacraft. Yes - more airmanship/lookout from him, a better FIS, might have helped, but.... If you are high wing / low wing or whatever, you still need to ensure you are looking out and covering ALL angles
Told you wouldn't like it
NoD