First series of D2, wide gauge tractor, 3J was narrow gauge. From 1938-1947 10,561 tractors were built. The serial number you provided appears to make the tractor to have been built half way through the series, likely to be an early 1940's tractor, and in your neck of the woods, probably a really nice find, as one would think these are not so common to be found in the UK.
Hopefully some help, not sure how to tell the production dates on these with casting numbers or similar, but I believe those can be deciphered, I think I had it written somewhere and its tucked in one of the many old caterpillar manuals or paperwork I have in my collection, someone on ACMOC or ACME forums may recall. Best of luck with your endeavor, post some photos of it if possible.