I have to enlarge a cut out in my bathroom wall, it will be approx an 18” long cut, I was thinking thin cutting disc in an angle grinder with hoover for dust extraction, no water or elec to worry about, possible?

I used a diamond edged cutting blade in my Worx mini circular saw to trim some tiles that were already installed on the bathroom wall; not 18″ worth – perhaps 6″ in total.

It was much easier than I anticipated – no bother at all. It even came with the correct blade, which I only realised after going out & buying one (obviously).

A solid diamond disk in an angle grinder would be my choice,

https://www.screwfix.com/p/rubi-porcelain-diamond-tile-blade-115-x-22-2mm/5077k

Diamond cutting disc and tape a timber slat to the tiles with duct tape as a guide….

Good tip that ^