I suspect the others didn't account for the fact that your £400 includes the court costs to file the claim.
Well unlike the others I'll have a go at answering the question as I have experience of this. In the small claims court the worst that can happen is you will lose your claim completely and have to pay the admin for any counter claim, about £120 last I knew. They will not be able to hire Bob Mssingbird and get full costs etc.
Now if you can demonstrate that you lost a days pay as a direct result of their action, I'd go for it. In reality the judge will decide what they owe you plus your admin costs so even if you do not get the day's pay you will get the minor exes such as phone calls and interest.
80% of small claims are found in favour of the claimant. In my view this is just heavy weight bluster. Go for it, once they know you are serious they'll settle out of court unless they have a robust defence.
I have experience of this and believe me the judges that resolve these are very good at getting to the nitty gritty so be sure of your ground. I applaud you for taking this action, these ticket outfits need a good kicking sometimes.