The aircon in my Elegance is not getting cold..

As it's only 18 months old I'd guess it doesn't need re-gassing and this is (sort of) backed up by when I turn the AC on and off the engine doesn't drop/increase revs.

Any thoughts before I speak to the dealer??

It will need re gassing.

Just had my wife's fabia re gassed at ATS. It is ice cold now. Cost £45.

The air con was not cold at all before and making a hissing noise.

The aircon in my elegance 170 does not have any impact on engine revs when turned on. I'm not sure you can use this as a diagnostic?

Don't they need regas/top-up every 2 years anyway?

My wife's Yaris A/C has not been regassed and still works after more than nine years.

Same on my wife's 10 year old Honda Jazz.