Subform DataSheet ignoring(?) function?????

[TOPSie]

I have a form:-


Which is a Form with a with a subform – here is the design view of the subform



Which is a Datasheet
It worked well with the first 7 columns each with a single field as Control Source.
I have added an eight column which is far more complex. It uses a public function (in VBA) to calculate an 8 week average based on the vehicle and ProductId from the same table which is the main source of the (sub)form
The Control source of the 8^th column is

 =gcAverageEggReturns(GetEggVehicle(),[ProductID],GetEggDate())

I have put a break point in the function and I know it is obeyed when the form loads and I can see it returns non-zero results.
But when I run the form normally the eight column is always filled with zero.
At some point I did observe the form load quite slowly and the first 7 columns appeared immediately but the 8^th took longer to load as if it did some sort of repaint and lost the function results in some way

Any ideas what is going on????

[Minty]

Put the function call as a calculated field in a query based on the source table for the form.