I doubt you need a suit, but would imagine it is shirt and tie always check there website l am sure it will make a mention of attire on there or always call, l went to Midsummer House which is a Michelin 2 star and never wore a jacket and felt really under dressed.

Let me know what the food is like as l am thinking of going to Les Maison Quat ses Son or what what ever it is called.

Here you are, no ties required

http://www.the-ivy.co.uk/index.asp?area=52

Mike