I actually put a brand new (2018) stem on a Mk1 (c 2001) when I lived in Lancaster! I bought it from Fudge Cycles in London and I had no problems at all. All components were compatible. Perhaps you have one of the really old Mk1s (c1995) which had a thing like a seat clamp on the stem, but I recall that somehow that all fit together too. Of course, if you do need a new headset or want to upgrade [the original headsets sold back then in the UK were pretty average] that bike shop at the lower end of town in Lancaster can do it. They were used to me coming in, and even rebuilt 18 inch wheels, somewhat reluctantly. I used a LitePro Birdy headset to upgrade. Still going well in Australia, where the weather is substantially better than Lancs/Cumbria of course. Simon