Ended up removing 2 rows of tubes at the tank ends to get good access, cleaned up and putty is applied, rock hard in 10mins as advertised, and now nearly fully cured - just need to put it back together and give it a test.....

The allisport rad is 3-1/2 years old, but I have only done 1000miles with it, so not overly impressed - unfortunately its well out of any guarantee.....