The part number for the 80w heatsink is KJ6KH. If you would like to purchase one you can try Dell sales @  08000854961 and see if they will give you a quote. 
I have a 3240 compact with an i7 10700 (TDP 65W) that absolutely pummels the standard heatsink. HWINFO regularly shows the cores hitting 100oC and averaging >85oC when running a full virus scan. The service manual shows an 80W heatsink is available on some models, but I can't find that on sale online: Precision 3240 Compact Service Manual | Dell UK Are there better, alternative heatsinks / water cooling solutions out there for this device? Thanks.
Can you upgrade the heatsink in the 3240 compact?