My mechanic more or less rebuilt much of my A/C. At the time, it leaked oil from the interior hoses. Most of those parts are near impossible to find. Every blue moon a google search will turn up something listed here and there at some AC specialist, but not in stock.
With the Diavia units, I think the key thing to remember is that these, in themselves, were kits made for post-production vehicles. Many of their parts may seem unique to the '94 Defender, when viewed (and sought after) through that angle. But when a good mechanic looks at each piece, many can be subbed out with near identical parts labeled differently. I can't remember all that was done, but I believe the drier was almost identical to a Mazda unit. The compressor is a Sanden unit. The expansion block came from somewhere else, as mine was cracked. Diavia put these kits together using many parts made by suppliers common to other makes.
I tried this route, rather than getting frustrated and ripping the whole unit out (and spending $3k on parts alone). Good luck.
"Uhh...I think they all leak like that."