+1  A: 

If any Aggregation (Grouy by, Sum, Count, ...) in the Query or any joined query you can't edit the data.

You can write an function the sum for every row in the table.

Andreas Hoffmann
Aha, I've already tried to put a function in the continuous form I'm using. It's a shame it's a bit laggy in fetching the result that way, but oh well.
skerit
I'm actually surprised that using an aggregating SELECT for a single column of a SELECT statement makes the other fields unupdatable. I wouldn't have expected that myself. Have you tried the DISTINCTROW predicate? I doubt it will help, but it works in some other cases, so is worth a try.
David-W-Fenton