I think I have managed to get it to work using a different "blank" character and I just copy and pasted it in place of the # symbol. U+2000 En Quad is what it says in the character map! You are a legend.
♫$repeat( ,$sub(3,$len($num(%track%,1))))$num(%track%,1) 
%artist% %title% $regexp(%_length%,^0(\d:),$1)