600x9 appears to be a standard size for fork lift trucks. Got to be worth a try, trailer maker will almost certainly have used a standard item from somwhere.
Saw 2 of these last week at RAMCO army surplus stores at Croft, Skegness, Lincolnshire . Dont have their phone number but a quick Google should do it. Postage should be no problem as they often put items on ebay with delivery.
Cheers mate /forums/images/graemlins/wink.gif I think you might have hit the right nail on the head! Looks like "A128" could be what I have, the tyre also which is a 6 ply.