I've been told by a local Defender 90 300tdi owner (I've the same beastie) how to fix an intermittent power loss problem I have on my '94 beastie which he had also experienced.
The power occasionally died right off slowing me to a crawl if lucky. After a rest the problem resolved itself with a cough and a splutter and it could be a few weeks before the next occasion.
His story was (unbeknownst to him) loosing the foil top of a fuel additive seal bottle in the tank and that occasionally blocked the fuel inlet. I've used additive and as I can't find any other issue I thought I'd have a look.
I got myself a spare cork gasket (ESR3278) as I'd have messed it had I not, cleaned up the area to keep the sh!t out of the tank (toothbrush and WD40), loosened the five screws and guess what I saw below the pickup. The offending bottle seal!
I'll have to fabricate a gauze cage from a stainless tea strainer for a coarse filter as there's nothing in evidence but I'll do that at a later date.
I used a triangular needle file to catch any stealth bottle seals hiding out of reach as it wouldn't be able to seal by cutting a wee V in the tip.
Kit needed, head torch, grabber, spare gasket, long cross-head screwdriver (screw nearest the door is a bar steward).