Piffleworthy, I don't think brute force is needed, but you do need to narrow down some options. Since you have 26d you know the first digit of 26a (which = O). There are four options. One of those options is more likely for the first expression in 26a (though the others are not theoretically impossible). This will give you a likely value for UM.
It pays to list the numbers in numerical order and fill in the letters as you go along. At some point you will see coherent bits of the message appearing, which helps to place other letters approximately.