Thanks for the reply, what exactly is it called? I tried to search online last night for one but can't find what it actually is!
I have exactly the same issue with exactly the same cistern so thank you this is a big help. New problem which I’m guessing the original poster will have encountered by now as well, you can see from his original post that the compression fitting from the flush valve to the down pipe is caked in caulk. That’s also true on my unit and I’ve seen several posts about leaks coming from down pipes on high level units all over this forum, all of the fittings are also covered in some kind of sealant. The reason (at least in my case) is that the brand new down pipes being sold by the likes of Victoria Plum and also found on eBay all seem to be 1 3/8” or 35mm while the standard flush valve fitting is 1 1/2” so there’s a gap. To make matters worse, these chrome down pipes come with a bell trim that fits over a smaller than standard compression nut so you can’t fit a larger washer in there to make up the difference. I’m at a loss for how to fix this short of soldering a 1 1/2” collar around the top of the chrome pipe to make up the difference but that will probably discolour the pipe. Any thoughts/ideas greatly appreciated.