Silicone caulking will do the job. Another option is a rubber stopper. Use rubber cement to keep it in place.

Bathgate, thankyou so much. I was afraid that I would be advised to find another container and I do have a few, but really prefer this round one.
I'll check out silicone caulking; hope it had the same name in the UK.

It's readily available at your local hardware store or DIY. Good luck with this project.

we tend to call it silicone sealer, use it to grout between tiles and the top of the sink/bath.

or you could put a circle of old carpet/blanket etc in the bottom and then that will 'reduce' the chance for the liner able to bulge through the holes. It would also be useful to put carpet/underlay/blanket around the sides to as an insulating layer. This will reduce the amount of freezing in your tub.

Since you're using a plastic sheet, I would use a rubber stopper and plumber's putty, just in case the plastic leaks.

Thankyou SBG and Tug. I’ve ordered some silicone sealant and should receive it today. I think I might adopt your suggestion of old carpet and put it under the plastic. Plumbers putty sounds just the job but as I’ve ordered the silicone now, I’ll go with that for now.
I’m burying the tub, or most of it when I can dig (or if ) a big enough hole. So the ground will insulate it in winter.
Thanks again all.

I didn't register plastic sheeting, plastic will go brittle in a few years. If you can get an off cut of a pond liner ie butyl liner it would be better.

Oh crikey, that's something I hadn't thought of. More evidence of how important this site is. Thanks SBG, I'll have a look on ebay; still not going out into the world.