I need to change the flush unit in the toilet cistern. Unfortunately the gap between the top of the cistern and the shelf above is not wide enough to get the new unit in. How would you approach this? I guess it’s either removing the shelf (not sure how easy that will be) or do I take out panelling behind the toilet and in front of the cistern etc and hope I can then remove the cistern so I can fit the new flush unit in it (a lot of work). I attach a photo. Many thanks