A real DIY question

I havent had it a lot, but in less than ideally heated and ventilated houses it always seems to start there.

Well that's another way of doing it. and solves the immediate problem of sealing by at least preventing anything other than torsional movement at the bath edge, but to me, a bath that i can push the sides outwards while sitting in, even if its fully supported from below, just feels tacky. And in the case of my baths, they are not always mounted TO the wall anyway. I wanted a ledge to put the soap dish on. It isn't hard to treat a cheap B&Q Or Victoria Plumb piece of s*it with a fivers worth of scrap board and car body filler and make it feel like a £600 top line acrylic bath, and solve all it's flexing issues.

A few cans of expanding foam. Although I wonder if the problem might actually be the floor rather than the bath.

Although I wonder if the problem might

That was the problem with the floor under the bog when Im purchased a property 40 years ago:)

Half a decade of a leak or misdirected pee :(

Last time i did this i filled the bath to past overflowing! put the silicone in let it go off then emptied said bath, so bath is pressing up to silly cone worked very well!...