chair loose covers shrunk

not enough space here for all relevant details. had company(UW) supply material and make loose covers for 2 armchairs. specifically stated need to wash covers approx 3 monthly and they advised suitable material and washing instructions. after 2nd wash iaw instructions cover shrunk so bady that it can't be put back onto the chair. company fetched the chair on 21 April saying they would make another cover. since then i have phoned almost weekly basis. each time another excuse. finally 10 june received report from fabric company via UW saying that i had used excessive heat on fabric - washing, tumble-drying, ironing which is not true AT ALL. i have never had direct dealings with fabric company yet UW say that i should have it out with fabric company. i told them that i did not think that it was up to me to do this. UW undertook to provide material and make covers and should therefor sort it out. over last 3 weeks approx i have started demanding my chair back several times but still do not have it. two weeks ago it was promised by end of last week. this is surely wrong on their part, it has been extremely inconvenient and i am getting nowhere with them. what do i do please?