My son has a 1999 Corolla SE 1.3 which is the 4 EFE engine. It has covered 71,000 miles and has been in our family since new.

Over the last few months it has developed an engine misfie between about 1400 and 2000 rpm. I have done the following to try to cure this problem:

Battery charging checked
fuel injector cleaner added to fuel tank
Clean air filter
new spark plugs
new fuel filter
new high tension leads
coils replaced ( second hand but checked by supplier)
Fuel injectors checked for clicking usuing stethoscope

Can anyone suggest anything else?

I am unable to find the lambda sensor, if it's got one it's well hidden!

On most occasions engine misfires occur due to engine timing. This is especially the case if it's a misfire in the exhaust. I would check that first and possibly the Oxygen sensor on the exhaust, but make sure to ensure the engine is timed first, sounds like you already put a lot of money in the engine... don't want you to spend more if it's not the cause. :)