Modifying "Cost"

When you say that the summaries have a "lump sum" cost against them, can you be more specific.
What Task field is the cost in and how did it get there?

Have you typed numbers directly into the Fixed Cost field (second column in the Cost Table)?

Have you typed numbers into the Cost field (4th column in the Cost field?

Have the Summaries picked up a cost from a Resource assigned to it?

Regardless, you should not use the Summaries for anything other than headings and sub-headings for groups of Tasks and Milestones. ie, no resources, no costs, no fixed costs, no predecessors, no successors.
You are better off having a dummy Task with a Fixed Cost.

Trevor,

From your comments it looks like the data was added in the "fixed cost" column which can be modified. I don’t know why this was subsequently copied across into the "Cost" column.

I take your point about having a dummy task to assign a cost to rather than using the summary.

Thanks for your input

Bob

The behaviour and the logical reasons for it is easy to figure out with a small example.
Make a few tasks, indent them under a summary (heading), don’t assign any resources.
The reason you cannot change the Cost field of the summary is because it is a calculated field that rolls up the costs of the tasks. You can type directly into the cost field of the tasks. MSP will assume that it is a fixed cost because it doesn’t come from any resource cost, and will fill in the fixed cost field of the task for you (very helpful).
You can type directly in the fixed cost field of the summary.
The total cost of the summary is the rolled up total of the tasks plus the fixed cost of the summary.

Here is one reason not to put in a fixed cost (or anything else) for the summary. When the Analyse Timescaled Data in EXCEL wizard in the analysis toolbar only searches for the costs of tasks and skips the summaries. Easy to test.