2002 Rover 45 coolant needs refilling

Temperature gauge goes up. Coolant needs filling up - not consistent though, sometimes empty really quickly, sometimes lasts for long time. Hose pressure checked and no problem found. Garage says may be head gasket but reluctant to tackle it - but I would prefer to fix before it goes bang and I am stranded with knackered car! Sometimes noticed bit of water spluttering from exhaust but not loads. Engine noise changes, chugging sound and car feels 'lumpy'. Any suggestions please??