@ Simon.....

Reduced Reverse Speed (% of maximum speed)

The 1208C controller can be specified with a reduced maximum reverse speed.
This is a safety feature designed to prevent full speed in reverse where vehicle
control may be restricted. A 60% maximum reverse speed setting is typical and
is a good compromise between performance and control in reverse.

http://www.junktronix.com/curtis/curtis.html
has some info that you may find usefull.

Curtis in Northampton could check your controllers settings.