How can i tell what type of boiler system i have ?

I have a baxi boiler approx 20 years old with no manual. I have no idea what model ! I am trying to suss how to fix a hot water problem and i am being pointed in the right direction with regards to looking for a valve ! I do not know if it is a Y or S system or even gravity and believe the difference in system will result in a different repair ! I have been under floorboards looking for valve and cannot find it ! When under floorboards i noticed both pipes from and to are scalding up to about 2 metres from boiler ! tried to clear sludge from pipes and i do get a steady flow of water if i take pipes apart ! So i believe sludge is not a problem ! Pipes then go progressively colder before they reach my hot water tank ! Could the valve be behind my boiler which is wall mounted or will it be nearer the hot water tank ? Any answers would be greatfully appreciated as the quotes for fixing it differ by hundreds of pounds !