prop vac line has been pulled off or split. or need to to smoke test the boost side on inlet pipe work,, ie between turbo and inlet manifold,
if not these two things then divert'r valve or turbo problem maybe ,, if car was running fine before working on it then something is loose or split or pulled off i would guess

 

below link might help
Tech Info and Help - 2885 Boost Deviation Plausibility Error - JM Turbo Coopers

 

Thank you... I'm not mechanically minded... Where will I find it... Thx

 

its not a it, its more of a load of things that can cause it, the fault code you have is a outcome of a fault, ie that code has many things that will cause it, vague list below,
1,ecu problem
2 wiring to and from sensors to ecu,
3 vac leak
4 vac controller
5 turbo failure
6 divert'r valve on the turbo faulty
7 waste gate on turbo faulty,
8 car in limp mode due to other problems,
9 inlet pipe broken/leaking
10 oil pressure low stopping turbo from spinning correctly
and few more
without a proper diagnostics its near impossible to pin point any thing going by just having that code. all you an do is go back through all you have moved and see whats been broken or split or left off maybe,

 

Just had a good look around and can hear a hissing sound when I turned the engine off from around the right hand cover of the valve cover or turbo...

 

Just had a good look around and can hear a hissing sound when I turned the engine off from around the right hand cover of the valve cover or turbo...

 

sounds like you have a leak on the pressure side pipework, or pcv valve and breather pipe work maybe, thats where a smoke test helps,, you could use a can of easy start and very short bursts around where you hear the noses with car on idle engine note changes then it means its sucking in the easy start fluid and thats where the leak is,, but remember its very flammable,

 

I also noticed the hose at the top right hand corner of the valve cover has no clip on it, will this affect anything?