From April - September present my car has always ran 1.6bar with an odd spike to 1.7bar.

Today this morning on my Defi's it spiked past 2.5bar and went even further on the guage. I didn't even notice till I pressed 'peak' and saw it.

Now when I boost I hit 1.4-1.5 boost at most.

Any ideas whats gone on here? is this that multiple advancer thing taking control to protect my car?

 

It will probably have detected knock and adjusted timing, I'd be more worried about why it hit 2.5 bar, before worrying about only getting 1.5.

I take it you've checked all the piping?