Ok guys, a little update.
I replaced the boxes + checked the original cable (all fine), put it all back on and...very little difference.
I also noticed that selecting a faster wiper speed does nothing, just the same lethargic speed as the slower setting.
I've looked at replacing the park motor switch but that seems to only affect where the wipers stop?
The brushes have been mentioned to me as a good place to start, what do you think?
Thanks again for your help!