I'm upgrading an old pc from a socket A athlon processor which i believe is an 1800+ to a socket A Athlon 2400+. Can i use the same heatsink / fan?

Most probably. You'll have to clean the bottom of the heat sink of any thermal paste that it might have on it from the old CPU. Usually a citrus based cleaning agent does the job well. Then apply new paste on the 2400 and away you go. It would be wise to watch temperatures and if Bios settings or motherboard software allows for it adjust the fan speed if needed. But from memory, the heatsinks rated for 1800+ should be sufficient for 2400+ too. Can you tell or do you know if its the OEM heatsink that comes with the processor or a after market one? As I've answered assuming that its the stock OEM one that you have.

Its the stock one that came with the 1800+. Thanks for your help