HI Otherside! Haven't 'seen' you for ages -- you still HTTSKWL-ing? It's working more for me now than when he was younger.
Anyway, I don't think this will go with fish pie, but I love roasted veg with olive oil & basalmic vinegar, roast for 45 mins, then add feta cheese. I do this either with a mediterreanean theme: courgettes, aubergines, mushrooms, red onions, potatoes (par boil first), bell peppers, cherry tomatoes (add at same time as feta, only final 20 mins), or a sort of 'orange/ yellow' theme: sweet potatoes, potatoes, baby corn, onion, leek, carrots, butter beans. Add herbs & seasoning as you wish. Very nice in pita bread the next day too.
For your fish pie, I think I would do a nice crisp salad (with home-made vinnagrette) and peas, poss the leek dish just mentioned?
Good luck with your dinner party.