MARK GROUP. A company to steer clear of

Mark Group quoted to change my boiler, it was all going well untill the first tradesman turned up to start the work. He asked where the boiler was going, so I showed him. It cant go in there he replied, but dont worry I will speak to the supervisor when he gets here. After about a 1hr discussion after The supervisor turned up, they decided to put the boiler in a totally different place to where it was planned to go. They are now telling me that although they used less materials, it still cost more money so wont be giving me any money back. I signed up to a fixed price job and took advantage of the interest free credit with Hitachi Finance, the quote which I was given clearly stated that they would replace the flue, but in the end they didnt need a flue which was £552. They also fitted an 18 KW Boiler instead of a 24 KW boiler, apparently only a difference of £22.78. Instead of a refund they maintained the complete job is another £80.00
Tried talking to the customer care team, who basically dont give a DAMM. HOPOEFULLY PEOPLE WILL READ THIS, please Steer away from this company.