surface mount shower to plasterboard wall - fixings???

[tgs166507]

Hi. I need to install a shower on an external wall which is plasterboard dot and dabbed on to blockwork. as you'd expect, there's a 1" ish gap between the PB and the blockwork. The wall is finished with tiles, and i'm not going to take the tiles down until I redo the bathroom entirely in a couple of years.
In the short term, i'm mounting it with exposed pipework with some proprietary chromed elbows, but i'm slightly worried about the robustness of the fixings.
I'd like to drill and fix into the blockwork, but then i'm worried about damaging the tiles/plasterboard by overtightening the fixings, and essentially drawing the plasterboard towards the blockwork.
is there a trick of the trade for this?
All I can think of is to try and put some expanding foam (or ideally something that sets more solid) into the fixing holes in an effort to fill between the PB and the blockwork immediately behind the fixing points so that I can tighten them up nicely without worrying about the damage I describe above.
Has anyone any suggestions?
Thank you in advance.

 

[Madrab]

ideally you would fix it through some of the dot n dab that is behind the gyproc, if there's some in the position you wish to fix the shower. if you tap the wall the sound should change when you're at a piece of that, from hollow sounding to solid.
Apart from that there's nothing I've come across that would fill the void injected through the fixing holes. What kind of shower are you installing? You could use gravity toggles or butterfly bolts to fix?

 

[PrenticeBoyofDerry]

you can get 25mm hollow wall anchors, if the void is slightly smaller, drill through and it to block work, to allow the anchor to be placed in to void. You can also cut the screw down if required, using junior hacksaw. As the wall is tiled the plasterboard grip will need filing down, so it fits flush.
Also buttterfy and toggle fixing can deal with this.
If you decided fix through to wall, then you could squeeze some gripfill in to void to help support the board.