Just pay it. You can appeal but will get nowhere, meanwhile the clock will be ticking and the higher amount will be due. Then their solicitor will send you a letter and suddenly it’s £200 plus due to their fees. At that point it’s either pay or get a CCJ against your name.
Used to be you could challenge and win until some fool took it to court, defended himself, it became case law and now everyone is screwed. IANAL.