Okay, I've duplicated your problem. My first reaction was to create a character variable before running the report (from outside the report writer), and then use "On Exit" on the detailband to update the string:
cAllFields = cAllFields + field3
But it didn't work at all.
Second attempt, same idea, but in "On Exit" of detailband, call a function. For example, fieldConcat(field3), (where field3 is the field you are trying to add together). Make a function to keep track of your growing string...this works fine, and has no (practically) limit to the string size.
>Have created a report variable (character) which sums string values over multiple records. It's too short to hold the sum in some cases. How can I get around the apparent 255 character limit, please?
Steve Gibson