i think you should go back to the garage ,do you have a reciept saying wat they charged you for ie redex or fuel cleaner ?. they have no right to do anything to your car or add nothing without concent.
no mate the heater tank is up stairs the bath and toilet where both down tairs separate the downstairs lav had hot and cold water so i tapped in directly above where it went through the floor/ceiling to run into new bath room hence being 15mm .
just moved in a new house where the bath and toilet has been dow stairs i have a brand new boiler and heating tank pressure down stairs is fine ive just fitted a bath and sink up stairs and managed to tap into the water hot and cold before it goes down stairs ive run in 15mm for basin and 15mm...