I just replaced the boost diaphragm in my 2.5TD for similar issues. Only instead of the turbo boost pressure bleeding through and pressuring the fuel tank as you report, mine was allowing fuel to flow through the boost line the other way into the turbo cool side and then into the intake manifold. Under power, it would bog a little, and then knock and ping like crazy under heavier load.
After several days of troubleshooting prior to figuring it out, I eventually had a 8-10 second diesel runaway after turning off the ignition as the truck rapidly burned off the diesel that had pooled in the intake. This also likely wiped the bearings in the turbo, blowing oil through the hot side.
The truck had a newly built/remanufactured engine as part of my purchase, and the seller is a super stand-up guy and is replacing the new, four-month-old (less than 150 mile) turbo for me. When I swapped out the boost diaphragm it was pretty clear that it had been pinched when the injection pump had been assembled and eventually developed two large holes.
The Delphi replacement diaphragm part # is 7243-006, and it's worth you also reading the following post if you haven't already: Replacing 2.5TD boost Diaphragm - Land Rover Technical Archive - LR4x4 - The Land Rover Forum
If you have the ability to wait a bit for it for shipping from the UK, by far the best price I found for the was here: http://www.delphipartsstore.co.uk/fu...iaphragm_3.htm
. They were really quick, though. I think about a week from time of order via USPS/Royal Mail.
I guess many diesel shops and tractor repair-type places may or may not stock them here in the U.S., but I just played the hermit and searched the web.
If you're stuck somewhere in immediate need, contact me and I can try to get one to you. I bought a spare for myself.
If you start having any knocking or pinging like I did, you need to stop until you get this fixed and potentially do some serious damage.