Thanks Toneman.
I should have searched more thoroughly before posting a similar problem. Sorry about that.
Forgot to mention that to stop the volume fluctuating, I've increased the min pedal position in the edit menu to 100%.
This stops it jumping between volume levels but now I can't use the volume pedal and the wah still has a restricted range, doesn't fix the problem but allows you to use it at a constant full level.
Looks like I might need to replace the pedal circuit board. Not sure how to get a hold of one though.
I'll try the solutions offered in that thread when I can and post back results.
Thanks again Toneman, cool dog.