In my post above I meant to put 33 x 4, 4.5, 5. This was from Longstone Tyres, where they showed 33" tyres as being suitable for 24" rims. They do however list various 32 x XX tyres for 23" rims, see here:
http://www.longstonetyres.co.uk/search?searchMethod=searchTerm&searchScope=site&searchUrl=&sortBy=&category=&searchTerm=1061&x=18&y=5
I'm sure they will confirm if your rims are ok at 3/8 below nominal size. Straight sided tyres may well need this tolerance for fitting.
I have a M5 bomb trailer which came with 18" rims, but with one 18" and one 19" tyre - the previous owner had been using it on the road for years with heavy loads! :cool2: Good luck!