Tbf it is a common area to go and cant be noticed until the crossmember is removed for gearbox work....just had the same issue with mine after i thought i had tackled all the chassis
Would seem to be a common problem....sold my 300 engine to a guy whos engine had done the same, my advice wouldbe buy a mot failed 300 disco, bought mine for 250 engine sold for 350.... Other bits on car should cover the cost of fitting etc.