Help with gas boiler?

our gas boiler wasnt working last oct and the landlord got his pal who fitted the boiler into check it but didnt have a clue so had to pay for actual engineer which he wasnt happy with. the engineer fixed the prob but said the boiler didnt suit what it was put in for as if we get hot water in bath we cant get it same time in kitchen for example. obv the landlords error for getting incorrect boiler. So when the heating is on and we turn on the water the heating turns off until water is stopped when it then resumes. if my heating is off at the boiler and on water only i get warm water in tap but for some reason for washing machine to get warm water i need heating and water on. Sooo confused, any ideas? Were in private let