Shower valve siting issue

[Bob Nelson]

Hi, I'm new to the site and am hoping someone can help with a bit of an unusual problem. I recently bought a house and need to replace the shower. The one that the boss wanted leaves a gap around the valve when fitted. It looks like the present one was a diy job because the holes for the feed pipes behind the valve shrouds (hope that's the right word) are way too big and are barely covered by the existing one. Another couple of mils and the shroud fittings would go right in, and they are quite large to start with. So the new valve had to be moved to one side slightly to enable me to make it secure with the fixing screws. This obviously leaves quite a gap to the other side of the smaller shrouds. The shower cubicle is more recent than the rest of the bathroom and has the multi-board type walls.
We are not without a shower as it is quite easy to rig something temporary up when needed, but I really need to sort it out.
Thanks for any and all help. Bob