Task Entity ID

Hi Everyone,

Probably a really stupid question but I'm having a brain fade at the moment.

If I'm on the task form I can find the id of a task in javascript using

var sGUID = Xrm.Page.data.entity.getId();

But how do I get it if I call the Xrm.WebApi.retrieveMultipleRecords("task", "?$select=xxxxxx") from another entity?

Does anybody know the ID fields name? I need it so I can update things on the task should things change on a custom entity it's linked to like if the owner is changed on the custom entity the owner of all the uncompleted tasks gets changed also.

Thanks

Frosty