I have been offered to buy a set of Genuine Lexus 18" wheels that came off a top of the range IS model, I have the basic model that came with 16" wheels as standard and Lexus tell me that the 18" will not fit, however they could not tell me if I could buy anything that to add to them to make them fit such as spigot rings (if thats what you call them) could any one tell me if it is possible to get the 18" wheels to fit my model?

I don't see any reason why 18" shouldn't fit.

Should not have any problems.
Spigot rings (Should not be needed if they are genuine Lexus wheels as the centre bore is the same ) But if you do need them they are only cheap and take literally seconds to fit. Plastic or alloy.

Think lexus are telling porky pies.

offset should be 45