Incomplete address - Return to sender?

[Emiry]

I ordered an item from a UK online vendor on Friday. He dispatched the item the next day. On Tuesday, the track and trace service changed to 'HELD AT ENQUIRY OFF' then 'RETURN TO CO'. I rang up Royal Mail and asked them why it had been returned to sender and whether I could come and collect it. The advisor on the phone told me it was already on its way back and the reason was that the address was incomplete. I rang the vendor and let him know the situation. He confirmed the address I had given him and told me that they just print the label directly from the information entered online. The address was 100% correct, with house number, street, post code etc and no typo errors. He said he would send me another package of the product I'd bought out straight away but now I'm worried that it too will be returned to sender.

My first thought was that I'd got the suburb incorrect - the address sits right on the border between two and they have been used interchangeably for other mail that gets delivered here. According to RM postcode checker though, the entire address is correct. The only thing I can think of that has gone wrong is that the address label got damaged so it became illegible. What are the chances that the second item will not be delivered either?