Seems like the lamp heat has damaged the holders , you may need to get new holders fitted or maybe new fittings.
Sometimes it is not best to use 50 watt lamps if fittings are constantly on.
you can also get lamps that prevent heat going back into the fitting, but it sounds like the damage has been done
Ok so I thought one MR16 bulb was pretty much like another. I did say go easy on me. How was I to know I got the wrong type? That's carp design to my mind, that you can get an (apparently) compatible bulb that will ruin your fitting