Thanks so much both. I've checked and my bike is a cross race sl - maybe 2016 rather than 2015 (sorry!). I attach the spec I found on the web. I tried to buy the ST-RS685 which sounded the perfect solution but unfortunately they don't deliver to the UK.

So I think I'll go with the 180mm rotor, especially as I have the other new bits anyway. I've researched rotors and found an RT-MT800 for £30 which sounds ultegra level spec (says for MTB but fits road/gravel. Plus another post to flat adapter so only another £40 in total.

Sound the right decision?

Thanks again