The CSDU on the F4 was indeed a nightmare to replace. The comment made about changing it in a bay situation is absolutely correct.
We found this out when a couple of Woodhall Spa engine bay lads were posted on to the squadron (111) and were astonished to find out we were expected to do a CSDU change on wing.
However, I found the bigger CSDS on the 1-11 easier as it was slightly more accessible and the stand supplied was replaced by 2 fitters.