Fault Code PO123 - Came about after Exhaust change from stock to 3"

[mudski]

The fault code will reset / erase when I goes out , but it comes back again.

The boost was bouncing all over the place up over 20psi sometimes , I put a boost controller on it and it now doesn't go over about 13psi

I did notice earlier that it takes a bit longer to get the revs / speed / boost up when taking off, not much happens till it gets up to about 3000rpm.

Fuel / Fuel filter / air cleaner etc have all been changed

???

That would be the low boost. 13psi is waaaaay to low. your EGT's would be through the roof. I would set it to 18 atleast. Add a needle valve in with the boost controller and this will enable you to control the spoolup rate of the turbo too. I just googled P0123 and TPS was the first hit...