Ian is your guy on this. He introduced me to the fact the the 1500B block is essentially the diesel version of the gas MG block. It is NOT a ford. It is British Leyland through and through. I was recently able to purchase reasonably priced gasket from a British car parts supply house for my W30.
And... this is important: The W30 is NOT the 4-91 in any way shape or form. The W30 replaced the 4-91. I recently rebuilt mine. Photos are on flickr dot com slash chuckbullett. It is a sweet sounding engine when you have everything dialed-in.