At a guess what has happened is that the existing washers have dished due to the clamping force of the tap. If you fill the existing holes and redrill to the correct size the washer would work
You can fix fibreglass (the base of most "plastic baths" but it's a pain in the bottom
There's two way.....the "professional" way and the semi easy but maybe good enough way
The proper way involves a lot of work and is best done with the bath removed so that you can pour gelcoat.
so on to the easier way lol
Go to your local chandlery. They will have everything you need
First off you need a sheet of fibreglass cloth. about 2ft square would probably work but I'm sure you can figure out the size when you see what's involved. You'll need about 4 rectangles as described below
Close weave mat is much better than the chopped strand stuff as it's easier to work with
You'll also need fibreglass resin (a 2 pack glue type thing) to go with the mat.
Also you will need some gelcoat, colored to match the bath and some microballs suitable to make the gelcoat into a filler.
If you explain what you're doing the chandler should know exactly what you need
Prep work
rough up the bottom surface with a bit of med grit sandpaper - about 120 grit
Chamfer the top edges of the holes at about 45degrees maybe 3-4mm additional radias
Then as carefully and exctly as you can mask around the outer edge of the chamfer
First step - basic fibreglass repair
Cut 2 recangles of mat such that you cover both holes with about 2" overlap around the holes.
Mix up the 1/2 the fibreglass resin. Coat the underside of the bath around the tapholes. Make it good and wet with resin. Then apply one rectangle of mat to the area. Using a paint brush stipple the mat into the resin that's on the bath. Then apply a little more resin and a second rectangle of mat
STipple into place again with a little more resin
Leave it to cure, probably about an hour but check the MIs for recoat times
Cut another couple of fibr cloth rectangles about an inch bigger than the first setand repeat the process
That's it for fibreglassing. Remember to wear the correct PPE and keep the windows open. Resin is awful stuff although it will be quite pleasent
When all this has set you should have a structurally sound bath again but it's still got 2 effing great dents in it. Gelcoat time
Step 2 - Gelcoat
Following the MIs mix the 2 gelcoat parts. Then add the microballs to create a gelcoat paste. Using a spreader spread this into the top of the tapholes. leave it slightly proud of the old surface. It will take a good while to cure
If all goes well you should end up with 2 raised circles the same color as the bath. Now's the really tricky part. Using progressivly finer grades of sandpaper you nead to work on the gelcoat until it's the same level as the existing bath. Be very careful. You don't want to scratch the existing finish.
When you get close to done switch to wet and dry paper, ending in about a 1200 or 1600 grit to add a shine to the surface
IF all goes well the final finish will not be perfect nor will it be invisible (the microbeads in the gelcoat will be visible) but will not really show unless you know it's there
Re-drill the holes to the correct size and refit taps. You can re-use the plastic asher idea to cover the repair. This time it will not leak
If you're really brave -
Leave the gelcoat/microball level a mm or so below the level of the bath. Allow it to cure then add a layer of pure gelcoat. Do the finish work on this
