A person you owe money to is entitled to add interest on the debt, and the current interest rate is 8% - that rate is set by the court. Any person/company who issues a claim through the county court will have this generic piece of text added to the claim.(posted 8354 days ago)It is possible for the judge to refuse interest, that will come at the hearing. If you get a summons then defend it.
As to costs, all I know that is some types of summons have a fixed rate for solicitors costs, others the full amount. You can get more information on costs in a court leaflet which you can download from the court service website - www.courtservice.gov.uk