felicia overheating

Hi,

I have a 1.9 felicia diesel which is overheating. I've changed the thermostat (but I'm no mechanic, no may not have done it correctly). I have leak in the cooling system which is somewhere around/ under the heater matrix. I'm still driving the car, but only for short journeys, and I keep the heat down by driving with the heater blowers full on hot (and the windows down!). The fan doesn't seem to cut in - but should the fan be cutting in on normal driving in this weather? And the heater blower on full..

I can't think what to do next - other than taking it to a mechanic..

Any suggestions gratefully received!

Thanks.