WHAT'S WRONG WITH MY HALSTEAD ACE?

my water started running hot cold hot hold for weeks then all of a sudden the heating went, I tripped the overheat button on the boiler and the heating started againg but the water is still running hot cold and if water runs for to long i.e A BATH! it trips out the heating? I had the heat exchange replaced 2 months ago because I was told it was that. It worked for a month...of course the company now gone into liquidation!! help..??