another coolant issue

2 garages have looked at this now and the mg solutions guy, has been pressure tested, the cap to the coolant tank replaced twice, leak stuff put in, the mg solutions guy found a leak from the pipe connecting to the tank was a little lose, may have been the problem. its been fine for over a month until last night light came on and complety empty ??? this morning after topping up last night light comes on while driving, then goes off half way into journey, god this is really annoying me, going away tomorrow and need to use car for a 300mile journey and its a sunday no garages open, really feel like driving it off a F*****g cliff (without me in it ofcourse)

any ideas ????