replace it

Turns out I need a 245/40 R18 93Y to replace like for like (even though the front tyres are 97Y??)

I'm assuming these are not the original tyres for the car as it is very unlikely it would have come from the factory with tyres of a different weight rating front to rear. As long as the weight rating of the tyres are not less than it should be for the car its fine. My VW Caddy van has a GVW (gross vehicle weight, i.e, the heaviest the van can legally weigh fully laden) of around 2200kg and I would generally use 91-95, so I would say 93's would be fine (but check if you want what the factory fitted). Using a higher weight rating than needed may adversely affect the ride a little (it isn't a big difference after all), but it shouldn't be a problem otherwise.

I haven't looked at your pics, but if you have any doubts about the tyres, replace them.