If the court found in favour of the defendant, I think they might be able to claim some minor costs, e.g. for travel, although having said that, hearings are usually held at a court close to the defendant. But as has been said by others, if you win, you can claim all the court related costs from the defendant, and some other minor costs, like the aforementioned travel. When you make your claim, don't forget that costs are in addition to the main claim, and don't count towards the maximum £5,000. Costs are sorted out at the hearing, so you don't need to guess what they'll be on the claim form - just put the figure claimed 'plus costs', and have the costs listed when you go to the hearing. Don't forget you can also claim interest on the payment owed.
I would advise getting some free legal advice (there's a telephone number on the court forms, plus some home insurance policies include legal advice, or through Which? etc) so you are confident about the basis of your case. It's very helpful if you have a good paper trail to back up your case, and a coherent argument. The basis for the judge's decision in the small claims court is what is likely to have happened 'on the balance of probability' rather than 'beyond reasonable doubt' which is used in other court systems (i.e. a less strenuous test).
There are books around and information online about representing yourself at Small Claims, and it's useful to look at other similar cases (although I don't think you can access Small Claims cases online).
Good luck, and don't be put off by the defendant's legal representation.