Simply because all Boiler replacements need to be of the Condensing (High Efficiency) type, very few Mexico's are probably removed still in working order, as most Folks who are correctly advised will hang onto them until they are completely dead, and no sensible Gas Engineer will fit a secondhand boiler anyway.Pray do tell why they do not get sold on ebay for vast money - No engineer wants to work on them - The ****ty engineers want to replace it with a combi. The better ones want to pull up all teh floorboards and repipe the whole. The best engineer talk about valiant heat only boiler and adding a motorised valve
Best advice with the Mexico is to find someone who knows these Boilers, (they are very simple in real terms), get it Serviced annually, and make sure the system is correctly dosed with Inhibitor to prevent internal corrosion.