There is nothing I can think of that fits the bill so if you have to stay in Lewes you might be able to get someone to make up an icecream sundae to order. www.famiglia-lazzati.co.uk/ is an italian / pizza resturant in market street that go the extra mile. It gets very busy on weekend evenings but they might put something together if you asked in advance. Otherwise Bills (Cliffe High St 01273 476918) is known for being helpful and already do their fabulous deserts and smoothies. The real eating company (cliffe high st as well) know what they are doing and at the far end of the street is a friendly cafe behind an antique shop, I cant remember if they do ice cream though. I hope you have a great time - its a great idea... childs face when they see a sundae with everything on... 3 times as much icecream than they can possibly manage.