My son has a 54 Ibiza but I've only used it with the bike inside with the seats down so I'm no expert. But I have used Thule bars on other cars I've had, the bike fittings that go on them are transferable. They do bars for Ibza's too. Like this
http://www.roof-rack-supplies.co.uk/thule-car-roof-bars/SEAT/Ibiza.aspx . I've not used this company, if you want a local supplier
Halfords sell Thule too. Not the cheapest but I've had my bars about 20 years on 4 different types of cars and used it for carrying a tandem so they seem sturdy.