Before I actually removed the sub frame I had already disconnected the calipers and removed the axles. I couldn't get enough access for removing the differential - well I didn't fancy scraping knuckles on the underside of the car to reach the four bolts that are attached to the sub frame so I finally managed to get the prop shaft out and pulled the diff and sub frame out together from under the car.
After easily removing the diff - and taking off the trailing arms I was able to fling the sub frame about with the greatest of ease for the next bit - bushing removal
Here is a picture of an attempt at removing the bushing by hammering it out with a bit punch
As you can see it didn't work. The rubber in the bushings was not strong enough to take the punishment. So I turned the sub frame over so that it was upside down from the position that it is fitted on the car.