1998 Citroen C1 Failing head gasket

Engine Cooling problem
1998 Citroen C1 Front Wheel Drive Manual 55000 miles

I have a Citroen Saxo 1.1 car and I recently noticed water leaking from the radiator. When I opened the cap to put more water in there was this gooey stuff on the cap. I took it to the garage we use regularly (they are really nice people there and have always been very honest and fair with us) and they had a look at it. They said the head gasket was failing and that it would cost around £600 to replace. I haven't got that sort of money to spend on the repair and I can't afford to buy a new car so they said that the good thing was that there didn't seem to be water in the oil and that I could continue to drive it as long as I drove it sensibly and checked the oil and water regularly. They said I should always keep a bottle of water in the car, try not to exceed 60mph and try not to get stuck in traffic jams which would cause the engine to overheat. Can you please advise me as to whether you think I should try and find the cash to get it fixed as it has a relatively low milage for it's age or should I just keep driving it until it gives up the ghost?