Can someone please help/advise what we can do next

[SimonG]

You just use the upvc trim to gap fill or create a tidy " border" so to speak?

Nice tidy border.

 

[ShaunCorbs]

Or maybe this

FloPlast White uPVC Quadrant Moulding (L)2.5m (W)12mm (T)16mm | DIY at B&Q

With a contemporary style to create a minimalistic look and feel to your home. Use the Square primed skirting to add a high-quality, decorative finish to your door frames. Thanks to the primed finish you can easily paint this skirting to match your home décor and style.

www.diy.com

 

[Knappers]

I'd imagine if it was a tight fit and expanded itself off the wall then the board manufacturer would be claiming 3mm as a minimum...?
Unless you've signed for a limited guarantee, just see how it comes out. Your plumbers liability doesn't end when he leaves site - far from it!