Client contesting invoice

Hello, I'm a digital video editor and recently took a freelance gig from a small production company. They revealed immediately to be unprofessional. The two producers gave me contrasting instruction for the same brief, made me do a lot of work for nothing, took very long time (up to half day) to check my work while putting pressure on me to do it urgently, sent me back home because they didn't have the right equipment / space in the office etc

We did not discuss a rate for this specific job because they just wanted to see if they liked my work and we could establish a relationship for the future. I have never discussed rates with previous clients unless they specifically asked me to give them a quote, so I didn't think to have to spell it out. My daily rate has been the same for the last few years (£250 to £400 per day) in the last couple of years and never had problems before - I generally invoice my clients at the end.

The job advert for this gig was stating "Starting from £200 depending on experience & skills" so I thought that would be in line with my rate and experience (I have 10+ experience as freelance, with public credits - a lot of long term contracting work). The job advert was also only for an easier type of editing, while they also left with me the more complex task of coming up with the story. Something they felt comfortable doing because of my extra skills.

I kept on asking for feedback every day, I generally do this at the beginning of a relationship because I don't know the taste of the client and I want to make sure I am on the right track and they are happy. This feedback was always late. At times even a whole day later.

I had mixed feedback from them, sometimes positive and sometimes negative, I tried to change the things they didn't like (Music tracks, graphics etc) which resulted in obvious delay and more work.

Despite the told me they were happy with the final version, they are not willing to pay for the invoice I sent them. They claim that it took too long and in their opinion it wasn't up to other editors' standard and they they clearly stated the rate to be £200 (although the wording says it "STARTS" from and it relates to an easier type of work.
I offered to meet them in the middle and cut my rate to £250 and detailing every single fault that it was their responsibility- but they say their position remains unchanged.

I would like to take them to the small claims tribunal (the total is around £2000) because they should not get used to take advantage of freelancers - but I fear I might waste a lot of time and energy and they might win because I didn't spell everything out.

Anybody here had a similar experience or any advise?

thanks !