My tips would be buy one dress that you feel great in that will do for evening, and one that you can wear with flat boots and tights for the day (if you like that look), a pair of jeans and a pair of black trousers. Plus maybe a couple of long cardigans to wear on top of plain vests, t shirts or blouses - don't try and fasten these but wear a scarf if you feel the cold. Lots of thin layers seemed to be more flattering than a big chunky jumper but looking back at the photos I think I was a bit deluded about how good I looked!
Also don't buy anything you need to wear heels with, even if you can be bothered now you really won't want to in a few weeks time.
If you can't find a decent maternity coat (and I never could) then just go to primark/hennes etc for a swing coat or cape in bigger size.
I found mamas and papas to have some good quality stylish bits, and I got loads from topshop - they just do stuff from the normal range but in maternity sizes so its a good place to go if you want something fashionable. Also their trousers and jeans come in different leg lengths. Hope this helps, good luck with the twins
LondonLottie....whereabouts in Switzerland are you? If anywhere near Lausanne or Geneva, you might want to consider the relatively short journey to France and visiting a fab little maternity store in Thonon (approx 45 mins from Geneva by car; from Lausanne probably easier to take the boat to Evian and then a cab on to Thonon).
Admittedly, it is 5 years ago since I shopped there myself but the store was still there last summer, full of chic french gorgeousness. Will try and find the name for you.
I loved Seraphine when I was preg. They have a good website but if you are going to London they have a shop off High Street Ken which was great as I got a good chance to try lots on. Their jeans were just the most comfy things ever and were even comfortable in the month or so post natally whilst my bump when down.