As RoadRider 400 as said inspect it thoroughly and if you are not really sure about tcrs or carbon bikes take some one with you that is.
£1800 is a lot of money to most of us, very few buyers may be able to have that amount of cash so the good news is bike no mater how good, may not sell very quick, i was told by a bike dealer he could sell 100 used bikes at £50 quicker than he could sell one used bike at £1000 so be ready to walk away rather than cycle away, just remember the motto buy in haste and repent for months if not years.
Good luck and want to see the sales receipt if seller can not provide one, even if it is his bike it may be older than he says any doupt at that sort of money and i would not buy.