Do you have a thermostat housing for the xk 120…on the shelf? I have 2…supposedly same for 120: …but they are quite different in small but important details, though they look the same at first…and fit the same way…onto the radiator, and the top hose inlet is the same…BUT…one has an inside diameter…where the thermostat/sleeve inserts…that is only 46.8mm…this is NOT open enough for a proper bellows/sleeve (sleeve is 48.2mm) thermostat with to fit…that same one has a little bit larger bypass port outside diameter. The other one that does have an adequate inside diameter for the stat…the bypass hose connection is smaller (is appx 20mm OD) and worse the connection is quite tapered on the hose connection; so that when the hose is fitted…of course it wants to just slide off. Also I see the bypass hose is a pre-formed hose…wider at the thermostat end…than it is at the water pump end…these bypass hoses seem to be unavailable…? For now I have used 2 diff size hoses with a hose adapter in the middle…this still does not solve the “slide off the taper” problem…which is aided now only in that the hose is a rather straight run…so it kinda has no where to go…: …it does not slide off…due to the other end …at least statically, but I’d guess that pressure at speed could…make it come off? I will come back here soon with some exact measurements…but see if you have a 120 stat housing on the shelf for some comparisons …I am in need of a bypass hose. 
Nick