Thanks I can see your logic but am missing something in my execution!
My selection is as here:
{ainvdet.layout_section} = "D" and
{ainvhead.salesman} startswith ["87", "86", "80", "75", "70", "69", "66"] and
{ainvdet.invoice_date} = {?Date} and
{salesman.name} like "*Tele*" and
if {?Selection} = "Group" then {ainvdet.product_group} = {?Group}
else if {?Selection} = "Family" then {stock.price_family} = {?Family}
else if {?Selection} = "Supplier" then {stock.main_supplier} = {?Supplier}
else if {?Selection} = "Parts" then {ainvdet.part} = {?Parts}