Almost finished the backer board and then will tape before using a tanking roll on product. Can I leave this hole "open" or does it needed sorted and if so how? The reason its there was to fit the cement backer board as the pipe that feeds the outlet to the right pushes out a bit.

 

Thanks

Some of the tanking paints also come with a roll of mesh tape. I would fill the hole with a low expanding foam, cut flush, mesh tape and then apply the tanking paint. I'm only a DIYer so take this advise with a pinch of salt haha

Thanks v much. Be good to get some more input on this as well.

To refresh this topic when I was looking at the Grohe instal video for the shower set you will see from the attached screenshot that they show two orange fabric seals..... The larger one for the control valve is included in their kit but the smaller one for the hand shower pipe outlet is not. I got in touch with them to mention that it wasn't included in the kit pack and they said that it's not meant to be. They say it's not essential and if you want to use one you need to source it yourself. I would like to fit a seal like this but a quick look online has me struggling to source one. Can anyone provide a link for purchase of one of these fabric type seals with a rubberised circle in the middle to go over the outlet? Thanks.

Try contacting these folk? 
 

https://www.tilingsuppliesdirect.co.uk/product/mapei-mapeguard-st-sealing-tape-corners-gaskets?attribute_option=Pipe Gasket 10-24mm&gad_source=1&gbraid=0AAAAADi7Txwhm5bQ3G4JvQUvTF7oz_nyn

 

(Random internet grab)

The hole / gap can be filled with CT1 and smoothed flat with a scraper, and left to cure. 
The tanking solution will cover that in 2 coats with ease. 
I don’t really bother with those collars / grommets, even when supplied.