C5 valve cover leaking

[agaafear]

So, changed spark plugs on C5 1.8 petrol. Noticed threads at ends were a bit oily. Also noticed rocket/valve cover had slight leak at gasket point. Then I tried the bolts which didn't seem overly tight. I tightened them slightly - now realise that was a bit stupid. So, I'm guessing I need to get the car to a garage and the gasket replaced before driving any distance. Any advice? Car has low miles and uses about a litre of oil every 1,500 miles. Slight whistling noise around top of engine.