I'm SO stressed - anyone got any advice? (re tradesman) Long sorry :(

[yomellamoHelly]

We have someone decorating our windows.
When I asked them to quote for the job I told them I wanted it stripping back to the bare wood and stone, since this hasn't been done in 40-50 years and is all flaked and generally a terrible state.
Got the quote back which said scraping back as necessary. Assumed this meant as we'd discussed.
The decorator has spent 1 1/2 days doing a poor job at sanding back the existing paintwork (he's onto a lost cause imo) and shoring it up with two-part filler (which apparently is fantastic stuff).
Obviously I'm very disappointed it's not the job I asked for but can live with it if they do a decent job. But they aren't.
As part of the quote there was reference to stage payments each week. Again no problem with that.
But I'm not happy with the work they've done.
Today I tried to not pay this stage payment for the reason I've given above. I was verbally battered by 3 men for half an hour who wanted two thirds of the cost of the job. My 2 boys were going nuts. So in the end I wrote a cheque for half the money (they asked for two thirds) and dated it for tomorrow on the basis that two of them were going to work this pm and two were coming for all of tomorrow. They also said they wouldn't do any glossing until they'd agreed the undercoat was to my satisfaction.
After that I was so upset I took the boys out to escape and collect myself again. It drizzled all afternoon and rained properly from 3:45 onwards (still raining now). I was only 2 miles away.
While I was out he (maybe they I don't know) undercoated everything in sight and gloss painted the bay and the double windows above.
What do I do now?
Feel tempted to cancel the cheque, but am terrified of the consequences of this.
Also wonder how good the paint job will be when carried out in this weather.
The main boss kept going on about how they've been doing this for 30 years and they're not charlatans. And that they have a 2 year guarantee on all their work, so I'm worrying needlessly. (Not sure what that actually is worth in the event.) And that he'd seen the preparation work already the day before (I went out for 1 1/2 hours) and it was more than up to scratch. - The implication being that if he was happy with the quality of the work then I was being unreasonable and had no grounds to stand on.
WWYD in my situation?

Do not stop the cheque - they can just sue you for doing that, and even if you have good reasons for complaining about their work, those reasons don't amount to a defence against the cheque being stopped.

It is basic practice never to decorate in rain. Even the paint tins say this.

I think I would:
i) call a halt to the work.
ii) Tell them (orally and in writing) that you are not happy with what they have done and how they have done it, and you are going to get a second opinion. Tell them that they bullied you into writing the cheque and they should not present it or you will sue them for the money back.
iii) when you have time to think, take a view about what they have done so far. Is it acceptable? is it what they agreed to do? If they quoted for scraping back 'as necessary' and you agreed that quote, this is not the same as stripping the wood.
iv) if you think they have done a poor job, take photos immediately.
is there a trade association they are a member of?
v) get 2 quotes from reputable firms to do what you asked for in the first place.
vi) decide whether you want them to come back and do a proper job, or get someone else in to do a decent job.