well according to the possible causes you should check the inverters first
the alarm 5 relates to the pump inverter, not sure why the steering had anything to do with it , i looked in the steer inverter alarm codes and it did not show an alarm 5 with that description, there was no alarm 5 at all, it jumped from 4 to 6.
In any case i would just check all the inverters
then check the motors , either way the problem will be in one or the other.
If you have another truck (same model and system in it) with a known good inverter in it try swapping it out and see if problem goes away.

That's brilliant info swoop. Thankyou

Just one thing if may say. The truck used to do this often when about 2 or three times a day. It would suddenly just stop and the alarm mod 05 would come on and it would do it every time you are turning the truck. It would never cut out if you driving in straight lines. But if you switch it on and off it would just reset and carry on working.

However now the fault code comes up with VMN low next to it. And it won't reset itself when you switch it on and off and the forklift is stuck in the middle of the aisle with the forks 15ft in the air

Does this info help you to determine which diagnosis I should try first.

Serial number is C435T02051A
Year of construction 2003
Model R2.0

If you require any other information please let me know and many thanks for your assistance. It is greatly appreciated.

Regards

can you give a serial number from truck dataplate?

on trucks with the AC2 controller: Alarm on Mod 5 is for hydraulic pump circuit
VMN Low indicates low voltage on the motor circuit
when you connected a higher voltage battery it just went High because it saw a higher voltage than expected.

according to the manual it says to check the pump motor and circuits
Possible causes :
a) There is a problem in the motor connections or in the motor power
circuits; check that the 3 phases are connected correctly;
check if there is loss to earth from the motor;
b) Fault in the inverter - replace the controller

I would check cables, fuses and make sure the UAV connections on the controller are correct.
I would pay close attention to the motor because it does have a sensor bearing in it, those have been known to cause problems when they go bad.
This sensor bearing is also sometimes called an encoder bearing, i did find in another section of the troubleshooting manual about the pump encoder troubleshooting and it describes the encoder error as an 05 alarm error.

It sounds like the encoder bearing in the pump motor may have failed triggering that fault code alarm.