The dress code is the stylish end of smart casual. If you're confident that you know how to dress to impress then you're good to go. If not, then here are some guidelines.
If you're coming straight from work you're fine in a suit. Try and match it with a good quality shirt and stylish tie - not the same old thing you put on without thinking most mornings.
Shave - Although you may think Brad Pitt's 5 o'clock shadow in Fight Club looked cool, bear in mind that you're not actually Brad Pitt.
Although old joggers might be OK for a trip to the pub, you should make more of an effort for speed dating - girls WILL look at your shoes and reach their own conclusions. Match your shoes to your outfit - if you're going for a funky look, wear some funky shoes (funky as in cool, not smelly...) if you're wearing a suit then wear some polished dress shoes.
Make sure that everything goes together - no patterns with patterns, navy blue with black. And if you're wearing leather shoes make sure that your belt matches.
If you're not 100% certain you can pull off what you're wearing, then you probably can't. A beige corduroy suit with an orange satin shirt can't be pulled off by anyone.
If a T-shirt isn't going to make you look your best, don't wear one speed dating, you're better off in a stylish shirt.