The key question on the MOWP is 'dishonestly'. A court must apply common sense.Not every refusal to pay a cab driver then and there and 'making off' is necessarily 'dishonest' . The court might interpret your actions either as not dishonest, or not proven so beyond a reasonable doubt.The evidence, from your friend, that the cab driver rang to get money is potentially valuable . You need a lawyer, to advise.( I can, anyway, see an argument coming about whether it's proven that it was the cab driver speaking) .It 'goes to credit. It suggests that the cab driver is not honourable, for one thing, but it goes further than that.It casts doubt on the cab driver's account. It may suggest that he knows he's in the wrong and is happy to take money rather than have his story tested..The signed note is valuable too, if you can get it into evidence (that's what you need a lawyer for). Signed 'on behalf of' is not 'signed by' so it's not strictly 'his' document but your lawyer may well get it into cross-examination, and into evidence, even to the extent that it's seen as hearsay (as an admission made by someone else).It calls for an explanation, as to why someone would write it.. What had the driver done or said, that that was the result? It's a bit of a coincidence that there's the conversation with the cabby and then this note following and the payment of the money. The simple payment, in itself, could otherwise be twisted and 'explained' by them as an admission by you.
Whatever the CPS says, a reluctant witness is a reluctant witness.One who tries to avoid court, or who contradicts his previous statement, might be seen as wholly unreliable in his testimony ( obviously it depends on the circumstances and what he actually says in court).