steel shed rusting 3 months after installation

So i got a steel shed from what id call a pretty well known company (they have a stall at the ideal homes expo) installed just before Christmas.
All was fine until one day less than 3 months after installation I notice the steel box frame under the door appears to be rusting. I left it for a while and noticed it appeared to be getting considerably worse. So i contacted the company to have the problem addressed.
After a couple of emails back and forth and me sending pictures of the rust they said they'd send someone out and could I leave the shed and side gate unlocked so they could fix it as I can't be there during the day. I never confirmed this with the company as I wasn't happy about leaving my shed full of tools wide open all day while I'm not there. I was trying to see if I could arrange someone to be in the house to no avail. Low and behold the day they were trying to arrange a call out I get a phone call from them saying they're there. I explained the situation and the guys said to ring the office and sort another day no problem.
That was months ago. Iv since emailed numerous times and been blanked by said company a number of times. I got one reply saying they hadn't forgotten about me and they'd contact me "next week" but heard nothing.
So what can I do? The shed cost north of 2 grand and really shouldn't be rusting after a couple of months of what was a pretty tame winter. There are a few other things I wasn't happy about in the way they finished around the door hinges, to say the cuts in the steel are rough is an understatement. I'd do better myself with a pair of scissors.
My biggest fear though is the box frame steel is rusting away under the shed and someday it's just going to give. The she'd is in the corner of the garden so you can't even access half of it so even if they come out to treat the steel they won't be able to do most of it. What can I do???