ok seeing as i am now aware of the answer i will share it to save any folks wasting time searching and googling, i have done the rectification late this afternoon and it seems to have done the trick
it is a well known problem apparently with td5 discos and the problem and fix are known
would you believe the 13th wire of a part of the loom is not used and the ends of the wire are apparently sealed with silicone, but over time oil makes its way along the cable inside!! as it has broken down the silicone sealant and then deposits all over the red plug on the engine ecu and many times works its way into the ecu
i took my ecu out and the red plug was swimming in oil inside and this is causing the engine to run errrrrratic
check yours and if this is similar then clean everything down with with brake cleaner, allow to dry and then put it back together and you will be fine
mine is perfect again!!!