Standard Small Claims Track directions provide for each party to serve on the other side witness statements and documents in which they intend to rely at the hearing. You say you have not received his defence, but if there has been a trial date set, he must have entered a defence (even if it is just a bare denial) otherwise there would be judgement in default. Have a look at the Defence entered to see what he says about it - that should give you a clue.
However, whilst you can object, the Small Claims track is far more informal than more expensive litigation and it is likely that the Judge will allow the other side to be heard, since he has complete discretion on the evidence. having said that, if you are seriously disadvantaged and they have failed to comply with a court order, you should be able to get the matter adjourned and have the other party pay your costs in doing so.
I would suggest you read Part 27 CPR
http://www.justice.gov.uk/civil/procrules_fin/ menus/rules.htm#part21
and the associated Practice Direction to give you some idea of the rules which you will be operating under.