Is there any advantage for me in upgrading from 40 to 80

I am at present on a 40/10Mb/s FTTC connection and I achieve a very stable 20/5Mb/s on a 750M copper link.
I am contemplating moving to a new ISP and so need to choose a new contract. For just a few quid more/month I can choose an 80Mb/s connection.

Would there be any advantage or disadvantage in going for the higher potential connection or would it just be a waste of money or even worse, cause increased instability?

Almost certainly not, you're running at the highest your line can support both the downstream and upstream...

A waste of money. The line will remain as stable as it is but you will gain nothing in terms of speed.

Many thanks.