Okay guys... so in conclusion and to tie up this thread here's the summary;
As originally posted problems that occurred; no boost pressure from the turbo, no white/blue smoke, no ECU error faults, no engine mngt light, basically no signs of an issue. The boost would stop working, turn engine off wait 5 mins, then it would be okay again (only did this twice tbh before its stopped working completely).
After many days of investigating electrical sensors and wiring looms (as all signs seemed to be electrical) it was indeed turbo failure. The very special 'nut' came loose seizing the turbo. However, no one knows how or why it didn't show any of the common symptoms nor how it magically worked again after!
After much deliberation, we initially decided to go with refurbishing the existing unit, with new internals etc. However, instead (because of fundamental flaws with this "version 1" of the TBi turbo), we opted for a completely new unit from Milan which subsequently is like version 2. Apparently these newer units don't suffer with the same seizing issue i.e. the nut etc so im hopeful that i now have at least a few months trouble free motoring!
As for the cause, again, nobody knows! The previous oil which may have been the wrong grade could be the culprit but given that turbo failure seems common on the earlier TBi's it could just be that her number was up (55k on the clock)
In terms of cost, im not actually sure as a warranty company fronted the bill. All i can tell you is that it was in excess of £1k (probably closer to £2k) but they'll send a statement over in the next few days so i'll update this for future reference to others.
After all this... my only advice to TBi owners would be that if your experiencing anything other than normal boost (which varies from gear to gear) i.e. temperamental, smoke, limp mode, strong smell of petrol from oil filler cap etc etc then get it checked by your local specialist asap. If it is turbo failure and a spline or other components end up inside the block then your bill could be considerably more expensive. Im pretty fortunate to have the warranty but many others aren't.
Finally, if you are unfortunate enough to have this issue, do not settle for anything less than the newer "version 2" unit. A refurbed older unit will more than likely seize again at some point in the future.
Thanks again guys for your help.