Hi Charlie. Could well be the Head Gasket gone again. Was it re-tightened after some miles last time? worries me that you have had to top it up regularly. You might have a leak- refill the system and do a pressure test-look for signs of leaks while under pressure. With system full, start engine, look into filler bottle for air bubbles while you blip the engine, or water level rising quickish-either is probably head gasket gone.Signs of whitish foam in camshaft area/oil might indicate same. If no signs,compression test on cylinders next step. Get head/block checked for cracks/level if you go for renewal of HG. Hope this helps.