On Mon, 22 Jan 2018 07:09:24 -0800 (PST)
Murmansk wrote:

I've damaged my fibreglass bath by dropping a heavy glass shower
screen onto it.

This photo shows the damage

https://www.dropbox.com/s/a07wjnz8hl...55.16.jpg?dl=0

I'm wondering how best to repair it - my first thought is car body
filler?

The best repair duplicates the original structure - glass reinforced
resin sealed with a gelcoat. You need to cut away the damaged area,
feather the back edge an inch or two with a grinder and lay up a
couple of layers of glass in resin (supporting it with a former
treated with mould release stuck on the front) then remove the
former when the resin is firm, key the edges of the front and apply
gelcoat (which you'll probably want to sand when fully cured depending
on how smoothly it goes on, but the gel just has to seal the resin,
anything else is cosmetic).