There is a fairly good chance that the claim would be struck out because Gladstones are pretty incompetent when submitting claims as they always fail to comply with CPR 16.4(1)(a), which if you can persuade a judge that such a failure justifies the claim being struck out, will work. You will be relying on the persuasive appeals judgments in CEL v Chan (2023) and CPMS v Akande (2023).
However, it is judge bingo whether you get one who is actually "persuaded" by the appeal judgments as they are not "binding". However, even f it were to go "all the way" and you were not successful, you would likely pay less than the claim amount as they have added a fake £60 or £70 which most judges do not allows in a small claims case.
Also, before it ever went as far as a hearing, you'd have an opportunity in a mediation phone call to try and haggle the amount down from what they are claiming. It would come down to how strong an argument you could put in a Witness Statement (WS) if it did progress all the way without being struck-out or discontinued.
I'd put the odds at greater than 50% but not as high as over 99% I would give in many cases. Decisions!