if I change this

select c.name, c.surname, c.telephone, c.lastorderdate
from customers c
where not exists (select 1 from orders
where customerlink = c.link
and recdate > '01-jan-2004'
and recdate < '01-mar-2004'
and branchcode = 'SAN')
and c.branchcode = 'SAN'
order by lastorderdate desc,1,2

PLAN SORT ((C INDEX (IDX_CUSTOMERS_BRANCHCODE)))
PLAN (ORDERS INDEX (ORDERS_CUSTOMERLINK,ORDERS_RECDATE))

to this

select c.name, c.surname, c.telephone, c.lastorderdate
from customers c
where not exists (select 1 from orders
where customerlink = c.link
and recdate > '01-jan-2004'
and recdate < '01-mar-2004')
and c.branchcode = 'SAN'
order by lastorderdate desc,1,2

PLAN SORT ((C INDEX (IDX_CUSTOMERS_BRANCHCODE)))
PLAN (ORDERS INDEX (ORDERS_CUSTOMERLINK))

the query runs in, wait for it, 3 seconds initially and 0,2 seconds for
subsequent queries :) - much more like the firebird I know.
I just dont understand why the index on recdate is not being used?

Thanks again Martjin, Luc, Daniel and Helen for your input

Regards
Gary