Spkid, let's get something straight right off the bat, I don't need your 'co-operation' to retrieve any money should I decide you are the guilty party here; and by the way, that is in no way intended as a threat, it is merely a statement describing the somewhat harsh realities of life.
OK, let's move on; I am not yet convinced you are the guilty party here but there is certainly some circumstantial evidence to suggest the 50 quid refund made was too high.
On that note, it seems to me, if indeed Zeb did offer that amount of money as recompense, then it tells me at least, he was a decent chap and did all he could to ensure you were happy .. in fact, he went over the top it seems to me.
That acknowledged, it would seem fair to me at least, if Zeb was kind enough to seize the initiative regarding the refund amount, then its incumbent upon you to reciprocate that gesture if only to balance out the deal.
And the basis for this suggestion of mine?
Ir's simple .. it he chose a sum that he believed would cover the damage repair and it's found out later that damage repair didn't cost that much, then you should mirror that initiative in evaluating best you can the amount of overpay by Zeb ... my opinion would be something like a 20 quid refund but I think it's much better for you to work out just how much he overpaid you .. after all, you might even conclude it was more than 20 quid but i think it's worth looking at this SPKid if only to keep the internet wolves at bay [that wasn't a threat by the way].
Just think about it please ... and let's see what you decide.