I sent my cheque made payable to Citibank. I shouldn't think they'd care who it was made out to - they cashed my cheque before the ink had dried!

I am starting a claim using the Money Claim Online small claims facility. I have hit a snag right away. They say that a PO Box in an address is not allowed and any such claim will be invalid at this first stage!! Does anyone know a version of the above address that does not use a PO Box number? I am keen to use this address as I have corresponded already with this person. Or is there another VALIDATED CitiCard address where a Small Claims legal document can be served? (As an aside, is there any evidence that this Box number is being employed purely to make it difficult to serve a legal document? You can't be too cynical!)