I'm always pleased to have another Brit here, even one from the red Dragon country ;). I am from SW england and London, but I live in Italy at the moment.
I hope the meal goes well. If your parents are very patriotic leeks make a great side with roast chicken, especially if you serve courgeets and peas too, and then roast potatoes, gentle and springlike, but very subtle flavours, you might like something a bit more special for their anniversary! Maybe find out what they ate on special dates, or what was served at their wedding...and make a simple version ;)
In omnibus amor et iustum