Andy, I've got a 3CIII with powertrain transmission and have had a similar problem. When the machine is cold, it will engage reverse but not forward.
First of all check the transmission fluid level. If it's not had a fluid change for a while, it might also be worth considering draining the fluid and refilling with a new filter etc.
I suspect that the clutch packs are worn. This isn't unexpected given the age of the machine. After checking the fluid level, it will probably help if you let the machine warm up for 5 mins before trying to move it.
Hope this might be of some help to you?
Steve
December 2009